Maybe they paid for the car with gold? Superman had, after all, gone mining for "fuel." One small nugget, traded at a bank for cash, should have been more than enough to rent a car and get whatever other necessities they might have purchased.
As for the baby, plotwise, he should have been crying and in quite a bit of pain. From a prodcution standpoint, however, I can't see making a baby cry or forcing a fussy baby to keep "working" instead of taking care of it. Besides, if you were that age and someone handed you a cool new glowy toy, would you have laughed, too?
Paul
