Imploded

I know it may be a bit of work, but if you download tShock, http://tshock.co/xf/index.php, and run a small server for your son only, you can eliminate all spawns.
When you get him ready to play, you just type into the console: maxspawns 0
No enemies will ever spawn.
I would like to point out that killing the Eater of Worlds is essential at some point in order to advance in the game.