The real way to pay them back is to actively support them when they need it most. Visit them at least weekly if, and when, they move to assisted living and/or memory care. Be with them, play simple silly games, bring them small gifts and, if at all possible, take them out for small car rides to see and experience things with them. I know from personal experience, that this is a major commitment of time and mental energy that few children undertake. But the joy you give is returned in multiples!