There's a bunch of old people that have kids that also very clearly don't give a damn about preserving the world for the next generation. Should they also not be allowed to vote?

Hell, I would bet that childfree people are more likely to support "future-preserving" plans than parents, on average. I'm thinking climate change, Fridays For Future.

Anecdotally, of the 4 sets of parents I know well enough to know their political opinions, none of them are really concerned about preserving the world for their children. I'm vegan, car-free, childfree and vote for the "green" options.