I was thinking of doing the samething for my 3 year old son. Couple Losi micro crawlers. I think it would be a good way to teach him how to drive, learning his left and rights, and throttle control. And its slow enough where it stays close to him and not runaway.
My main concern is durability. My local Hobbytown had one out of the box so I got to check it out. The guy said it had dropped off the table (glass display) cases to the hard floor around 50 with no problems, but the last one fell just the right way and broke the rear gear box. Now it only has front wheel drive. Another concern I have is what if your child tried to roll it like he/she would do with a typical hot wheels car. The tires are grippy and the worm gears prevents the motor from turning, so I could imagine some stripped gears or broken axles by a kid just forcing the tires to turn by just forcing it to roll on the ground. I'm trying to see if the crawler has a slipper clutch to prevent the gears or axles from if you just manually turned the tire with enough force. I tried to turn the tires on the crawler, but it was not slipping and I didn't want to turn it too hard where I might break it.
Those are my thoughts. I'm still on the fence. I was also looking into the Losi 1/24th rally car, but my concern about that is it might be too low to run nicely on my carpet.