Lol, I have a 300 SX and it is fun. Despite what others say at 190 pounds I have don't have a difficult time getting to plain and it doesn't take 45 seconds like some say it does.
On the other hand, I don't think it would be too easy to plain out of the water with addition drag. It's not necessary the child's weight, it's the drag. Not to mention, in most states what you are wanting to do is illegal.
Prob wouldn't be overly exciting for the child because all he/she would say is "Go Faster". But hell, who cares, try it.
Or just go buy a 4-stoke and be a super cool coucher and pull the tube going 70 mph lol