Armguards are an important (but not essential) accessory for successful archery. The purpose of an armguard is to protect the inner-lower bow holding arm from potential injury due to string slap at the time of arrow release.
Anyone who has ever felt the sting and burn of a bowstring will remember how it feels. Avoidance of injury is important for children (as it is for adults, too) and so an armguard is recommended.