The dolphin swim is the main attraction--if a child is too small to do it, but old enough to understand it, you will have some unhapppiness on your hands. The other activities are fun, but really you're paying for a day at a nice pool or water park for the smaller kids.
My daughter is 7, and swims pretty well, but it was something of a struggle for her to stay afloat during the dolphin swim, which was in deep, relatively cold water. I don't think she could have done it a year ago. If you're not a good swimmer, the flotation vest is probably a good idea.