If they're one of the 5 year olds that love animals, you could tweak Moon Druid to maybe have at will wild shape in place of spell slots. Being able to turn into any animal might keep them entertained and interested for extended periods of time, and a 5 year old isn't going to break a game with at will wild shape.

Just keep a couple good stat blocks on hand to apply to any Shape, that way they can contribute in combat even if they insist on being a camel all day long.