- Give them God's law - You must do it.
- Remind them they're sinners - You can't do it.
- Point them to Christ - He has done it.
- Tell them as believers - In Christ, you can do it.
Step 1: Give them God's Law - You must do it.
For example, let's say your daughter whines and refuses to help serve you in the kitchen because she'd rather watch T.V. You come to her and say, "You need to stop whining and obey your daddy by serving cheerfully right now. You must do it."
Step 2: Remind them they're sinners - You can't do it.
Your daughter looks away from you and whines even louder, "I don't want to." So, you tell her, "I know that you don't want to and I also know that you can't serve cheerfully and think of others first on your own. You're just like me, you're a sinner. You can't do it."
Step 3: Point them to Christ - He has done it.
Your daughter gives you a strange look. So you sit down beside her and say, "You know what, I'm so glad that God has mercy on sinners like you and me. That's why he sent Jesus. Jesus lived a perfect life and died on the cross in our place to take our sin and give us his righteousness if we would simply trust in him. Jesus never whined and always obeyed his Father cheerfully. He has done it."
Step 4: Tell them as believers - In Christ you can do it.
Finally, you look at your daughter and tell her that as believers in Christ we have been made new. We are clothed in his goodness because of his grace. Therefore, we have a heart that wants to serve and think of others first. It's in us by the power of the Holy Spirit. We can do all things through Christ who strengthens us. In Christ you can do it.
Realistically, you won't be able to delve into this deep of discussion every time, but this is the heart (and basic outline) behind how to approach these situations. Note: If you're child is not yet a believer in Christ, the Last Step is an opportunity to help him or her respond to God's grace offered to them in Christ - pointing them away from their performance to Christ's performance on their behalf. Remind them that Christ took our filthy rags, he took our failed test of obedience, and in exchange he gave us his white robe of goodness and his perfect score of obedience. We receive it all by faith in what Christ has done for us, not what we must do for him.