I let my nephew play on cruise servers sometimes. My little cousin who is 5 is actually really good at driving games in general. but a bit too small to reach the pedals to play LFS atm .
My 4 year-old drives occasionally, but he doesn't seem to get the "steer left, go left" etc idea yet. Either that or he just wants to crash. Given his personality, I'm leaning toward the latter. Kind of OT: A racing game he really likes is Racing Pitch - http://secretexit.com/games/racingpitch