"Is it normal that friends with kids, once they have kids, become harder to feel connected to? ... I don't know how to relate to their primary focus, and it's so much harder to find time to just get together. It's even harder to say, "Hey, it'd be great if we could get together without your kids," just so that we're not being interrupted every few minutes by a needy child. I love their kids, but don't want those to be the only get-togethers we have."
Carolyn Hax runs through the usual suspects: are you being judgy (your parent-friends are gonna notice). Are you being helpful (like, are you just another person demanding their attention or do you really help out with the kids). She quotes from an online discussion where someone suggest bringing ingredients to cook dinner after the kids go to bed.
I have a few things to add to this. The vast majority of my friends-who-have-kids had their kids before I had mine (I'm a late bloomer; okay, fine, I just never really grew up), so I was actually in this position for quite a while. I did, actually spend some time with a friend when her kids weren't around, but that was maybe a third of the time we hung out. The rest of the time, one or more kids was present. And she had, comparatively speaking, a ton of help: her mom was available, she had amazing sitters, etc.
I also spent time with a friend more or less exclusively as part of the whole family group. I spent a bunch of time babysitting both her kids (like, full time for a week or so at one point because of job stuff). And I still got canceled on because of kid issues. They couldn't afford a sitter. I was always wishing I had more time alone with her (we walked and swim together on and off as exercise buddies), but I never had trouble connecting across that kid boundary. I love those kids. They're like my other niece and nephew.
I also have a walking partner who does not have kids. She likes to think of herself as an auntie to my kids, and she buys them small presents when she travels or for Christmas and various other occasions. In practice, however, kids drive her binky bonkers. And that's okay, too. She's the best walking partner ever: 100% reliable and endlessly forgiving of my schedule.
On the one hand, I recognize needing to have things explained to one (I need to have a lot of stuff explained to me. Repeatedly. At volume. Despite my arguments that that doesn't make any sense at all). On the other hand, I am not sure I have ever met anyone who talked about the difficulty of relating to people who had kids (while being childless themselves) who was not, in general, a completely self-centered pain in the ass.
And I thought that _before_ I had kids. It was one of the reasons I made a point of figuring out how to maintain friendships after my friends had kids. I didn't want to be That Person, and at the time, I was by no means certain I would ever have kids of my own.
Don't be That Person. If you're thinking, gosh, my friend just doesn't have time for me since she had kids! you might want to engage in a little perspective taking. What would you have thought of that sentiment if you heard it from one of your own parents' friends. When you were in diapers, or an anxious 8 year old, or a constantly in trouble teen. I'm pretty sure you wouldn't have liked that adult much at all.