In the Lord's prayer Jesus tells you to pray for God's will to be done. By asking God for his kingdom to rule and his will to be done, you are making a life-changing request! You are acknowledging that his purpose and plan is better than your own. God's way is better than your way! It does not work to say or think, "Please do your will and mine." This is a foolish statement. Are your desires as important as God's? It is God's kingdom and will that must desired above all else. This thought ties directly to Christ's words just a few verses later in the Sermon on the Mount; "seek first the kingdom of God and his righteousness."
Jesus was faithful to what he prayed. In Christ's most challenging moment here on earth, he submits to his father and says, "Not my will, but yours be done" (Luke 22:42). Christ was totally consistent with what he taught his disciples.
So, when your child prays for a particular request, great or small, teach him what Jesus taught his disciples. His request may be for someone to win a particular race or event. Or the prayer might be for the hamster not to be sick, it may be for the sun to shine, it may be for Mommy to feel better, it may be for God to safely bring home a loved one from war. But in all of these requests, the most important thing, the best thing is to pray for God's will to be done.