My 5 year old uses something like this:
He's done GREAT with it! He's jumping in the pool this year, gone down the water slide. He loves it because it keeps him up after he slips down a bit under water. We actually let him swim in the pool with this on without us in the water (yes, we are poolside with him and watching!!) We are very protective parents and typically haven't let out kids swim without our hands on them, but he's done so well, we let him swim with his cousins as we talk and watch. I think it's a great buy!