I think there are two possibilities for Carly leaving on a plane. Either she is going off to college or her dad either retires from or is redeployed in the military and she goes to live with him. I guess Sam takes a motorcycle to L.A. to try and get a (paying) show bizcareer started, where she hooks up with Cat (who of course being from a performing arts oriented high school is also looking to get a gig in the entertainment industry). but of course before (or if it ever) happens they have to pay the bills so they start that daycare service.
But where would that leave the boys?