Second melosu on this one. I have had seahorses and they are very careful eaters. They like to look at each piece of food for a second or two before they will even thing about eating it. Clowns will wildly run around the tank stuffing their face with anything that is even close to food. It will freak out your seahorses.
Clowns can also get quite territorial. Especially when they pair up. They will at some point pick on the seahorses and stress them out. A recipe for a lot of sick seahorses and a very upset son when they kick it.
If your somewhat experienced in saltwater you can keep seahorses and pipe fish. Both are docile can be kept together will success most of the time. Both are also a little more difficult to keep compared to "nemo" which may have equal excitement value depending on how old your son is.
As a side note, under-gravel filters are outdated these days. they get clogged up quickly and start causing all kinds of problems that can only be fixed by tearing up your tank. Gravel in a SW
tank is actually not the best option either. Don't mean to come off rude or anything. Just trying to help.
Link to info on dwarfs