Are you keeping the game paused while looking for the child? It's best to keep it unpaused because sometimes kids can hide in the laundry room closet, they also disappear when they're behind the closed door in the toilet room as well as in the shower in the bathroom.

Also, if you unpause the game and then click on the detail screen and put it on the child's details, you can exit the detail screen and it'll move to the child.

Good luck!