2 kids I babysit (3 and 5 years old) like playing TF2 on my laptop.
For now I just let them play on training maps but I'll unleash them to the world very soon.
(tyler_rocket_shooting for letting them figure out basic stuff, tr_newbots or tr_arena_rc2 once they feel like shooting something that shoots back and moves)
Soldier is probably the best for learning how to aim. Pyro for learning movement
(might seem like a bad idea to make them play Pyro but I've noticed all other classes just make them stand still while they aim at things. Pyro forces them to move around while they aim. it's also the easiest class in the game to figure out. And they'll learn wm1 isn't always the answer once they start running into heavies and sentr nests)
Haven't let them play on online maps but I'd probably go with instant respawn servers on hightower, 2fort, mariokart, etc. You want them to move away from those maps eventually ofcourse but when they just start out that'll be easiest for them to get into