To be honest, C., it doesn't matter so much if it's not coast guard approved. You have to be in the pool with your kids anyway. And you need to be with them one-on-one, not 2 kids and one mommy. They are far too young for you to not be holding onto them or within a foot or so. So you are the life guard, not the guy or girl sitting up in the chair so far away from your children.
The flotation suits are a great idea because the kids are wearing them, and they'll most likely keep their heads above the water. The best thing you can do in the future is get them swim lessons at the Y. It's one of the best "gifts" I ever gave my children. They are excellent swimmers and feel so at home in the water now, and will for the rest of their lives, because we started when they were young. (3 and 5).
Have fun on your vacation!