The jumping and playing I saw at Orient Bay was a little intense for a 3 year old. The kids were bouncing each other into the water (about 4 feet deep). This was not quiet individual jumping, this was a free for all (not violent in any way, just frenetic). It might be different on other beaches.
There was also an inflatable slide attached to the trampoline. Kids would sit at the bottom of the slide while another kid jumped on the top of it. It would send the kid at the bottom flying high into the air and then into the water. It really did look like fun, but like I said, a little intense for the younger ones. Maybe it would be OK for the 6 year old.