I'd honestly be surprised if you found someplace that let children any younger swim with dolphins, just because of the insurance and liability aspects.
Dolphins are still wild creatures and can be unpredictable at times, so putting something that much larger and stronger than a child next to the child (who can also be unpredictable) and you could have a potential recipe for disaster.
While obviously every child is different (some well behaved, some not. some big for their age, some smal), they have to draw the line someplace.