Easiest method I've used with kids is to hand them a cardboard tube and have them look at you through it holding it in one hand. If they look at you through the same eye no matter which hand they are holding it with that is their dominant eye. Another way is to use a nerf gun or an unloaded bb gun and have them aim at your nose from both the right and left hand holds. It's pretty easy to see which eye they are trying to use to line up with.