Earlier quoted context omitted.
IMHO the part about the kids not being ready is crucial. If they're just not well off financially I'd see your idea happen (but then, we already see grand parents giving a hand financially in these kind of circumstances) But if they're not ready mentally, there would be mountains of issues with their relationship with their child. For instance, do they even fully understand it's their child under their responsibility…
My life was a mess until we unexpectedly had our first child. I don’t think I ever would have considered myself mentally ready. The child came first, then I started growing into a suitable person.
I don't know if I'm a suitable person, but having a kid sure pushed me out of my boundaries pretty far, in every possible direction.