Posing. I've never had a talent for giving direction to portrait subjects. I share the discomfort that 95% of humanity feels when a camera is aimed their way, and have no easy words to alleviate the unease.
I made a formal portrait of my mother twelve years ago. We drank a bottle of wine first, that helped a little.
When photographed, children typically are directed to smile. I think they get tired of that. I encourage children to anti-smile. Ah! A delightful new instruction!
Serious is part of childhood too.