I'm glad there are still parents out there who actually consider what their children are playing.
Alright, here's what you need to do.
In the .init file you can disable two things. the rent (look for no rent and set it too YES), and the Invaders (set them to NO).
Rent will means your kid does not have to witness dwarves emoing themselves to death over the economy when it activates, invaders being turned off will get rid of ALL goblins, kolbolds, and similar attacks. You won't have to worry about them.
There's probably a way to disable blood being visible also, but I don't know it <_<;
Also, do your kid a favor and install a graphics and tile set for him. I suggest Sphr's work over just about anybody else because of his level of detail combined with the fact he distinguishes between various sub-professions in a skill set. For example: Seige Engineers, Architects, and Mechanics are all considered to be "Mechanics" under the code. They usually appear to be the same type of dwarf, this is true even of Mayday (the other really awesome graphic set). Sphr's set differentiates between them.
You need to edit the text files a little, as well as the names you save the files under. For example, Sphrs_Dwarves.bmp should be how it is saved when you do the whole "save as" thing, you will want to change that too simply Dwarves.bmp. Further, the text file where you store the graphic's set will not be set to the right folder.
You will want to change the name of your folder from examples to Main, and then for the graphics sets for things that aren't humans, kobolds, goblins, and dwarves you will want to change THEIR file paths so it reroutes from the default folders (animal and creature) to main instead, so all your graphic sets are all nice and neatly packed away.
You'll have a really pretty set of graphics for the game, with easily identified dwarves at a simple glance. Everyone wins.