If you don't have ritual clothing, just wear what you would wear to regular UU services: in other words, presentable street clothes.
Whether children are welcome often depends upon the activity. The seasonal circles at my UU include meditations, usually, so when I had my 2yo niece with me I had to take her to the nursery until that part was over out of courtesy (it's hard to follow a guided meditation w/ a 2 year old making noise). But her presence certainly didn't offend anyone, and one of the organizers commented that my niece was more in touch with the point of the meditation than most of us adults were, just by virtue of her youth.
In short, it might be a good idea to give the organizer a call and ask before bringing small ones, but it's likely they'll be welcome as long as you're willing to retreat in an uninterruptive fashion if they become truly disruptive. If they're a young nursling, or old enough to at least sit quietly or play in the next room by themselves during parts that don't interest them, it shouldn't be a problem.
breastfeeding, babywearing, homeschooling Heathen parent to my little Wanderer, 7 1/2 , and baby Elf-stone, 3/11!