You have no idea how powerful these dolphins are...I remember when our trainer said to me "now, hang on" and that dolphin took off like a bat out of he double hockey sticks!!! 
You also spend a good amount of time treading water while the trainer is explaining things to you.  I kept drifting away, and it turned out to be a big joke between my husband and the trainer, but it just made me more nervous!
This is a great experience, but I wouldn't expose my child unless they definitely met the requirements.  Those dolphins may be adorable, but they are STRONG.  My kids were 8 and 12.