I would suggest there are no inflateable PFDs suitable for very young children. Children,by their nature, run into all kinds of things and don't think twice about it. Inflateable PFDs are easily damaged or punctured or triggered, and then they are worthless.
Better to put them in "waterski" type vests that are more comfy, and available in bright colors with their favorite heros or cartoon characters on them. At least you KNOW that will still work, no matter what the kids get into.