What types of questions do the boys ask? Here's my rambling thoughts.
Example: "Why do Uncle 1 and Uncle 2 sleep in the same room?"
Possible Answer: "Because people who they love each other generally sleep in the same bed. Uncle 1 & Uncle 2 love each other like mom & dad do."
I'd keep the answers very simple, let them digest that. Then when they are ready, they'll ask more questions.
8 & 10 are very much old enough to have heard the term "gay" before from kids at school, probably as a derogatory term (interchangable with "faggot" etc). Time to re-educate! These are great ages because they have the emotional maturity to start to think for themselves and weigh things out. There are also some good books out there for kids with gay & lesbian parents - check out www.twolives.com
for some book suggestions.
I semi-regularly babysit two kids who are 8 & 10 and their parents outed me to them when I got pregnant enough to "show" (I'm due in July) because they were beginning to ask questions about why I don't have a husband. (The parents already knew - I'd come out to them already when I got pregnant via donor insemination as a single mother by choice). The kids were FINE with it, just asked more intellegent/appropriate questions - about why I don't have a girlfriend, LOL, and does that mean the baby will get my last name and when can they see the baby?!
I also always highly recommend attending a few PFLAG meetings. I'm sure *someone* there has been in your shoes - how to tell children family members are gay, how to explain diversity, how to explain differences among people they love. They are a wonderful resource for advice. You can find a meeting near you by going to www.pflag.org
Oh, and tell the Uncles that the boys have started asking questions about their relationship. They may have some smart suggestions and definite opinions, too - any gay or lesbian person is used to "coming out" over and over again. It's a never finished process because you are always meeting new people and encountering heterosexist assumptions. It sounds like the Uncles are lucky to have you as loving family!!!!
And good luck! It sounds like your boys have a nice relationship with their Uncles. I think the truth will make it even better