I only used one once at Blizzard Beach, several years ago now - and all they asked for was my resort ID or credit card. I took out both and they took the KTTW card and there was no charge. Maybe I got lucky?
Yes they will fit I have a very tall nearly 6 year old however she is skinny Minnie but still fits in the XS. They have suitable for much larger children I think the go up to XL.
She is a strong swimmer now but I still put her in one to do the slides at the water parks as the currents can be strong in those catch pools
DD doesn't wear one in the pool but I do make her wear one in the waterparks. She swims well but currents etc make me feel better with her having one on. Most kids I've seen wear them.