This is a very simple boat your kid (or even you) can play with in a pool or a bathtub.
Tape a rubber band to the shaft, wind it up, put the boat on the water and release it.
You can print two boats to organize a race : the winner is the boat which goes the furthest !
Also, if you're interrested, join my discord server to discuss about 3d printing, arduino, processing, ... and find many interresting electronic project ideas : https://discord.me/electronic_projects
Don't use a too powerful rubber band : the boat won't move faster, but simply sink and the water wheels may break.