Children will sooner or later ask, "Who made God?" The obvious answer is, "No one made God. God is eternal; He has no beginning and no end." But this answer is not satisfactory for the children.
Explaining why it is so hard to comprehend eternity can help. Tell the children that the reason we cannot grasp God's eternity is that everything we see on this earth has a beginning and an end. Toys are bought new and eventually break and are thrown out. Pets are born and grow old and die. Some children have already had a grandparent that has died. Since God alone has no beginning or end, it is hard for you or me or the children to (not understand, but) comprehend this aspect of Him.