I've got to disagree with a lot of these responses.
I very much suspect you will be driving. At four a kid can manage a keyboard but is not likely to remember what key does what action. DF uses nearly the whole keyboard. Also I imagine he wants to join in and do something that dad likes to do.
As such, you will be able to determine, at the time of each game event, how to describe it. "That dwarf has gone to sleep, so this other dwarf is going to take him to bed." (Not to a coffin, understand, to bed.)
In fact, you should be giving descriptions like this all the time, simply to explain the normal events of the game.
"These dwarves are putting plant seeds in the ground." (Later) "See these? Those are new plants. Remember the plant seeds that were put there? They made new plants."
"We need to make more bedrooms for the new dwarves. Where should they go?"
As far as modifying the game to prevent violence, you should also have a look in the raw\objects folder. Some of the creature* files could be moved out of that folder to remove certain creatures from the game. At a minimum you need to keep creature_domestic.txt, creature_equipment.txt, and creature_standard.txt. I would also suggest keeping the "vermin" files: amphibians, annelids, birds, insects, other, reptiles, small_mammals. But the "monster" files like subterranian can go.
Guys, the parent knows best. If a kid is not ready to face the facts of life, let him be protected for a while longer.