Randy Rullamas, a real-estate agent in Fremont, Calif., bought himself a Tesla last year, and then another for his wife. He thought he might as well get a matching one for his sons, 8 and 3, too.

Theirs is pint-size, but it has almost as many bells and whistles as his Model S, with the same sleek lines, identical paint job and matching horn beep.

His...