The mother (who couldn't think of an answer) told her son to ask the stewardess. So the boy asked the stewardess, "If big dogs have baby dogs and big cats have baby cats, why don't big planes have baby planes?"
The stewardess responded, "Did your mother tell you to ask me?"
The boy admitted that this was the case. "Well, then, tell your mother that there are no baby planes because Southwest always pulls out on time. Your mother can explain it to you."