I'm in my early thirties and adamantly childfree. I'm lucky enough to be in a long-term relationship with someone who brought up her desire to be childfree on more or less our first date. But I am not having too much luck with my friends from childhood and university - they all seem to be wanting kids, and learning of their pregnancies leaves me with a feeling of sadness. I don't hate kids and think no one should have them, and I am happy for them if they truly wanted this, but I also know what them having kids will mean - we are essentially putting our friendship on hiatus, and I still don't know whether waiting 10 years for the kids to be a bit more independent and not requiring as much attention will mean I suddenly have friends again, but somehow I very much doubt it. And I also don't want 10 years without other friends than my girlfriend. She is in very much the same situation, and while we are good at making the best out of not having kids and stressing about having them, we both would want to be able to hang out with good friends once in a while, both common between us, but also some that are exclusive to each of us.
My assumption is that this is quite common - so I am hoping someone would like to share some success stories in turning this situation around. :)
I agree with the other commenter, over time you'll be the (hopefully) cool uncle who comes to visit.
Friendships grow and change, and these are going to be big changes. I won't sugarcoat it. You are going to see them less. They aren't going to be the same friends where they may have stayed out late or hung out for hours on end, those days are pretty much over - for several years. (They may join back up later when the kids are more grown, but it's going to be a while). They have kids and truly those kids will be their priority (as they should be).
For you, be available to them, know that you are still friends and make sure they know you are still friends, just know that it's evolving a bit. You may go over to their house with their kids to grab a beer, and they will probably be frantic. For you, don't be afraid to branch out from that group. Go out to drinks with work friends, join a club, whatever. They are still your friends, they're just going to be much less available.