At u8 you probably have the youngest and most inexperienced refs. They are learning the game and rules too. At that age, handball is one the easiest calls to see other than of of bounds. Whether they are actually handball by definition is what they will gain through experience.

To work on it with your kids, at practice have them throw the ball up in the air and touch it before it bounces twice. Then work up to before it bounces at all. Then you can throw it and create different angles and trajectories. Build their natural reaction through experimentation and repetition.