Re: PFD for children
for kids that age, I'd definitely have the type that have the rear part that looks like a "collar" of sorts and that have a grab handle on that collar. It is strong enough to lift them up from it. Also, they should have crotch straps, and you should use them. They keep the lifejacket from riding up on them if they're in water over their heads. It also keeps them from sliding out of the lifejacket if you have to grab them by the handle. BTW, the handle is very handy just to have as a way of steadying them when they're getting in and out of a boat etc, without giving them "too much" help.
My kids always wear PFD's in the boat and have never complained about them being uncomfortable. Of course we don't keep asking them "is the lifejacket uncomfortable?" either. (don't even plant the seed that there might be an option not to wear it) It's just a rule with us... the youngest has to wear a lifejacket whenever he's around the water, except when we tell him it's OK to take it off (e.g. we're in the water helping him learn how to swim). They've worn them at the beach for hours without complaint.
I've attached a pict... We just bought ours at the local sporting goods places.
Oh I forgot to mention... the small one will probably not float well in a lifejacket... They'll probably be too buoyant, and may not float head up like you expect them to (their legs don't sink)... Try it at the beach, and make 'em wear it, but you'll still have to watch 'em like a hawk.