And really, that could be just as interesting a discussion, if not moreso. Why are parents of adult children choosing to continue in a more "parental" role and continue dedicating their vacation time and dollars to their kids, rather than seeking out same-aged peers in their empty nest years the way past generations did? My grandparents would have been horrified by the way my mom is spending her retirement - taking my kids to animated movies and silly shows like Disney on Ice, traveling to Disney World, babysitting younger DD poolside in cities all over the map while I took older DD on campus tours, etc. They spent their retirement with friends from the Moose Lodge and the union hall, cruised and travelled with other couples, played pinochle together and golfed/went to the theatre separately with friends. Their vision for retirement wasn't nearly as kid-centric as that of the Boomer grandparents/retirees I know. I'm not sure if that is a generational difference or a familial one (the maternal side of my family is far more extended family-oriented than my father's side, but my maternal grandparents never really got a retirement because of health issues), but it does seem like there's been a shift along those lines since I was a kid in the 80s.
I don't think it gets framed as a burden, never mind a quid pro quo, when it is woven into the fabric of the family. Our family is very interconnected that way and there's nothing held over anyone's head. We want to help our parents and kids, and they want to help us. But when it doesn't work out, it doesn't work out - my mom has hired someone for some of the work on her house that my husband would normally have done because his boss recently quit and he's putting in too many hours doing double duty to have weekends free to do her floors. Because part of helping each other out and having the kind of enmeshed relationships we do is understanding that things come up and there's a natural sort of ebb and flow to these things. No one is keeping score.
So then how would you refer to offspring that have attained the age of majority? As "my adults"? Of course not, because "children" denotes the familial relationship the same way "brother" or "mother" does.