eventually over time and good water conditions, coraline algea will grow all over it , which might look pretty cool. IMO i would not put a saltwater tank in a 5 yr olds bed room....i can just see some curious little younging wanting to put something in there that shouldn't be in there...LOL
He helps me now with my 12 gallon. He knows to only feed a few morsels of food. Not to knock the snails with the mag float, not to put anything in and some things bite.
Sounds like a great way to teach a child responsibility and develop a love for hobby.
The sunken pirate ship would be cool. I'd stay away from any of the bubbling kind though.
Show us a pic if you get it set up and good luck!