What if they aren't eating enough because they're wasting it all?

I'd suggest making double what you think they'll eat and separating it into 2 portions. The first they can play with, and if some ends up in their mouths, bonus. The second one can be fed to them as they're playing or you can remove the "play food" first and then begin feeding them the other portion. Choose whichever way works best for your baby. It's true that they don't need much food depending on their age, but you still need to make sure that they are getting in their nutrients and gaining weight.