I'd rather the kids be interested in commercial aviation than, say, drugs or car theft. Kids in the flight deck don't concern me, as long as they're supervised by an alert, not-drunk, and sane captain (i.e. won't let the kid sit in the copilot's chair) and the kids don't touch flight controls. There are usually two pilots up there (captain and first officer) and sometimes 3 or 4, and the plane's running on autopilot once it reaches cruising altitude until the descent for landing. Frankly, other than the 1/100 million chance that a terrorist is on board ready to hijack the plane, a cockpit door that opens and closes once or twice isn't really a matter of grave concern.