Most children tend to discover this one their own, usually as they age or through friends at school. Considering kids can usually use a computer before they are in elementary school these days, a lot more information is available to them. If your child does come to you and asks you directly, it is important to tell them the truth, as this is a vital test to a child's trust in a parent and lying to your child is never a good idea. Make sure they understand that the deception was not meant to hurt or deceive to avoid bruised feelings. Most kids will ultimately appreciate the fact that they've been getting twice the presents as a result!