Maybe see if your local library has a age appropriate book with different kinds of families that you could read together and as you read along gently talk to your kids about it.
A couple I found on Amazon:
The Family Book by Todd Parr
Reading level: Ages 4-8
PreSchool-Grade 2-As he did in The Mommy Book and The Daddy Book (both Little, Brown, 2002), Parr introduces children to an array of families. Whimsical illustrations featuring neon colors and figures outlined in black show big ones and small ones, and families that look alike and relatives who look just like their pets. The art features both human and animal figures; thus, pigs depict both a family that likes to be clean, and one that likes to be messy. Some families include stepmoms, stepdads, stepsisters, or stepbrothers; some adopt children. Other families have two moms or two dads, while some children have only one parent. Interspersed with the differences among families are the ways they are alike: all like to hug each other, are sad when they lose someone they love, enjoy celebrating special days together, and can help each other to be strong. This concept book celebrating the diversity of family groups is distinguished by its sense of fun.
It's Okay to Be Different by Todd Parr
Reading level: Ages 4-8
(The Okay Book) combines rainbow colors, simple drawings and reassuring statements in this optimistic book. His repetitive captions offer variations on the title and appear in a typeface that looks handcrafted and personalized. A fuschia elephant stands against a zingy blue background ("It's okay to have a different nose") and a lone green turtle crosses a finish line ("It's okay to come in last"). A girl blushes at the toilet paper stuck to her shoe ("It's okay to be embarrassed") and a lion says "Grr," "ROAR" and "purrr" ("It's okay to talk about your feelings"). Parr cautiously calls attention to superficial distinctions. By picturing a smiling girl with a guide dog ("It's okay to need some help"), he comments on disability and he accounts for race by posing a multicolored zebra with a black-and-white one. An illustration of two women ("It's okay to have different Moms") and two men ("It's okay to have different Dads") handles diverse families sensitively this could cover either same-sex families or stepfamilies and also on the opposite page, a kangaroo with a dog in its pouch ("It's okay to be adopted"). He wisely doesn't zero in on specifics, which would force him to establish what's "normal." Instead, he focuses on acceptance and individuality and encourages readers to do the same
Who's in a Family by Robert Skutch
Reading level: Ages 4-8
Beginning with a traditional nuclear family and ending with blank spaces in which the child reader is instructed to "draw a picture of your family," this slight book catalogues multicultural contemporary family units, including those with single parents, lesbian and gay parents, mixed-race couples, grandparents and divorced parents. Kevin and his brother like their kimono-clad grandmother to help them with their jigsaw puzzles, while Ricky lives with two families. "Aunt Amanda and Uncle Stan," pictured riding in a blue convertible with their pets, "don't have any children at all" but are "still a family," says the narrator, because "they say Mouser and Fred are their 'babies.'" Because "animals have families, too," the text describes elephant, lion, chimpanzee and dog families as well as human families. (A human family headed by a mother is "like the chimpanzee family. Mama chimp raises the babies by herself, with the help of any older children she may have.") Nienhaus's lackluster illustrations, the schoolmarmish tone of the text and the comparisons with wild animals all tend to undercut the final definition of a family as "the people who love you the most!"
All Families Are Special by Norma Simon
Reading level: Ages 4-8
When a teacher asks her students to tell about their families, each child speaks of a different configuration. There are big, small, and extended families. Children live with a mom and dad, grandparents, two same-gender parents, or stepparents. The youngsters mention adoption, divorce, and death of a parent and pets. Then, they discuss the good and bad times that families have together. The tone throughout is upbeat and positive. The bright watercolor illustrations depict smiling, multicultural people living in immaculate middle-class surroundings. This is a good book for introducing nontraditional families to children, but some readers might find the sunny cheerfulness unrealistic. Ann Morris's Families (HarperCollins, 2000) represents a variety of family situations with color photographs of multicultural families and a simple text, but does not include same-gender parents.
Molly's Family by Nancy Garden
Reading level: Ages 4-8
PreSchool-Grade 1-To get ready for kindergarten Open School Night, Molly draws a picture of her family to hang on the wall-herself, Mommy, Mama Lu, and their puppy. After seeing the picture, her classmates tell her, "No one has two mommies." Despite her teacher's efforts to be supportive, the child is still concerned. That night, her parents explain, "we decided we had so much love that we wanted to share it with a baby." Thus, one of them is her birth mother; the other an adoptive parent. Still, Molly leaves her drawing home the next day. With further matter-of-fact reassurance by her teacher and the budding understanding that all families are different, Molly, and indeed the whole class, grows to accept her own family, and she proudly hangs her picture on the wall. While the children in the story are not shy about expressing their feelings, the author diffuses any tension by remaining focused on logic: Molly's family is as she claims. By tying this specific household to the general diversity within all families, Garden manages to celebrate them all. The soft colored-pencil drawings with their many realistic details depict a room full of active kindergartners. There is a squat sweetness to the characters as they work together to make everything look and feel right
Heather Has Two Mommies by Leslea Newman
Reading level: Ages 4-8
The story of Heather, a preschooler with two moms who discovers that some of her friends have very different sorts of families. Juan, for example, has a mommy and a daddy and a big brother named Carlos. Miriam has a mommy and a baby sister. And Joshua has a mommy, a daddy, and a stepdaddy. Their teacher Molly encourages the children to draw pictures of their families, and reassures them that "each family is special" and that "the most important thing about a family is that all the people in it love each other." In the afterword, the author (whose other children's books include Matzo Ball Moon) explains that although she grew up in a Jewish home, in a Jewish neighborhood, there were no families like hers on the television or in picture books. She came to regard her family as somehow "wrong," since there was no Christmas tree in the living room and no Easter egg hunt. Whatever the religious right may wish to think about nontraditional families, there is no denying that any child enrolled in an American school will encounter friends with single parents, gay parents, stepparents, or adoptive parents. This new, revised version of Heather Has Two Mommies offers an enjoyable, upbeat, age-appropriate introduction to the idea of family diversity. The book is essential for children (ages 2 to 6) with gay parents or family members, and a great addition to a Rainbow Curriculum.
) and mom1 sounded really nice on the phone. We moved this past year and it's always cool to meet new friends - for all of us. 
: on the books. Also wanted to add, we have a great book called "Everywhere Babies" which shows families of all sorts, and they LOVE it (it's a fabulous book too!). We talk a lot about it, but it is more of a toddler book... time for an update. 
I think this is a very common treatment for ANY "issue" book for children. When we were moving, we looked all over the place for books about moving to a new place, and they were all "she was scared the first day of school" and "he was worried he'd have no friends" etc. And I'm like, wtf???? These thoughts never occured to my kids. Also noticed this with books about the birds & the bees. A lot of them I've seen are "we know this is a really scary and really icky topic but..." 
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