Older children will do this naturally while trying to line up the sights. Very young kids may not get it, so another adult holding the rifle, adjusting for height, and aiming in the general direction can be a great help. Keep things moving. As they get more experience, give them more information. Make sure everybody hits something.
Some children are scared, because they think it's going to hurt.
Make sure it doesn't. Run everybody through several shots and let them choose a target. They will want to take home a piece of the clay birds, and any cards that they hit. At the Colorado State Shoot I run an aggregate after our practice rounds. The aggregate consists of three or four shots and a knife and hawk throw.