There's are obviously a ton of options out there. Go with bio lite or neoprene as they are a lot more comfortable than the nylon vests. More comfort=less complaining. We've. Had good luck with both the overtons and west marine store brand, as well as O'Neil and liquidforce. Fit and stretch are a lot more important than brand though.

Also, kids love jumping off the back of the boat. Doing that enough with a regular vest will result in chaffing from the vest slamming up into their armpits.

Depending on your comfort level, your 5 and 6 year olds might benefit from a vinyl waterski belt for those type of activities. I think 5 is about when we started allowing our son to use one for play.