I have to agree with Ms Jinks. I disagree with the gourami simply because they can be pretty skittish without live plants and with noise and such (at least the gourami I have had were)
I'm none too keen on a the idea of a three year old with a tank, But if your going to, go with a betta. I'm sure it would survive, and no special requirements. Not to mention, I love bettas, lol.
My 3 y.o. Grandson has 1 Betta and 4 whiteclouds in a 10 gallon tank. Daddy "helps" him clean the tank weekly. He is very protective of them and very aware about feeding them. It is shocking to think a 3yo can have that much understanding. He is absolutely devoted to them! Says "Good morning" to them everyday and "Good night" as well!!!
Get the kid a 500g reef tank.