Hahaha, Bernie fans of all people calling a policy unworkable/unrealistic because their candidate doesn't support it. That's hilarious.
So, there are a few layers to this. First, I agree that baby bonds aren't really the highest priority for spending political capital on. But if your goal is reparations, they're the most realistic option, bringing single payer into this is a full-on non-sequitur and your blinding hatred of a candidate making you throw out decent policy ideas doesn't help.
Putting that aside, let's look at your hypothetical and say that the bond idea gets canceled in 5 years time (And, let's be honest, any given healthcare plan will probably be getting dismantled piece by piece at the same time), if the legislation is written properly the bonds already issued should not then become worthless. So, at least you'd have 5 -15 years of wealth-starved babies with a head start on wealth when they reach the age of 18.