Visits from the cousins always equal fun times in the studio. Little ones usually have their own ideas of how they would like a session to go so as a photographer you just roll with it. A few tricks that have worked for me...up high equals a slower exit, squeaky toys sometimes provide a welcome distraction, always tell big sister or brother how helpful they are and what a great job they're doing, and utilize dad and mom to make silly faces right above the camera for a smile.
