HutchWilco make a great little one for babies. its all weight based not age i think 15kg. My kids had them well past 2yrs. It's got the strap between the legs to hold it down (that's all you need) plus the strap behind the collar to stick a boat hook in. Best of all it floats face up.
Next size up, note that the under- strap is not essential, we've found the side-grip ones (shown left and centre) grip just as good (better?) and more comfortable (and more availability). We know because we chuck the kids in from time to time so its fun and there's no surprises at show time.
Heres a selection tried & tested. Borrowed from us many times.