Since Katty and Lizzy are good friends, they of course enjoy each other's company, whether it be hanging out with their respective older sisters, sleeping over, or (in this case) blowing up their bellies into big balloons and floating away into the sky for a fun romp through the clouds. Nothing beats an excursion like that, after all.
Katty and Art (c) Joseph Staleknight
Lizzy (c) Joseph Staleknight and Marioblade64