Is this par for the course with grown children? Am I being petty?

My husband has four grown children, and raised three of them as a single dad. All three of them live on their own in the same town as we do and they all have kids. His boys each live with a significant other and his daughter lives with her kids on campus at the college she attends. The only time we ever from these people is when they need someone to watch their kids. My husband texts them fairly regularly and sometimes gets a response, sometimes doesn’t. I don’t try to communicate with them anymore because I have yet to receive a return text or phone call from any of them. They never hesitate to call me when they need someone to watch the kids.

It hurts my feelings that the only time they are interested in us is when they need something. It also makes me less inclined to help out. I know my husband would like to spend time with his grand kids, but truth be told when we are babysitting I am the primary caretaker because he can’t really do much. Helping out with my husband’s cares, taking care of my own school-aged kids, working part time, and getting ready to start student teaching full time has me spread pretty thin and taking care of these kids (all toddlers and some not super well behaved) is just exhausting.

So am I being petty here? Is this pretty much the way it goes with grown kids? Any ideas about how to get past my frustration?