The Lord's Prayer is found in Matthew 6:9-13. Sunday school teachers often teach young children to memorize this prayer because Jesus uses this as a teaching tool for how we should pray. We generally pray this prayer word for word, which is great, but it mainly teaches us what our own prayers should include when we go before God. Prayer is our personal relationship with God. The Lord's Prayer is our model of how to pray correctly.
If your goal is not to just have children memorize the prayer, but to understand exactly how Jesus wants us to pray, then this guide can help you. The translation below is taken from the New International Version (NIV).