These four verses admonish us on what God expects of us as the child in our relationship with Him. It is important for us to understand that "work out your salvation" does not mean work for salvation. It means we must take what God has given us to its logical conclusion. God liberated Israel from Egyptian slavery, but His purpose was not complete until they trekked across the wilderness and possessed the Promised Land. Their liberty came as an act of God's grace, but that same grace required them to meet responsibilities and carry them to their conclusion.
Verse 13 explains that God's grace did not end at the borders of Egypt for them, nor does it end for us once we are justified. He gives us both the motivation and the power to accomplish what His pleasure is for us. But we should understand that He gives us the desire and power to accomplish His will, not our pleasures. This is an excellent principle for parents to apply in motivating their children to respond positively. Children are largely the creations of their parents. If parents expect their children to reach certain goals, they must equip them with the attitudes, skills, and tools necessary to accomplish those aims.
"Fear and trembling" indicates both a deep respect for the Almighty who has called us, as well as a healthy measure of concern for uncertainties about what will be required of us as we proceed along this way. As we spiritually mature, the trials we must work through generally become more difficult, not easier. When the Philippians took up their cross, they did not know for sure what lay ahead, nor do we. For them, it was conflict (Philippians 1:29-30); for Jesus, death (Philippians 2:8); for Paul, martyrdom (Philippians 2:17); for Timothy, costly sacrificial service to the church (Philippians 2:20); and for Epaphroditus, physical illness nearly to death (Philippians 2:27).
Of course, these things are far more serious than a child's responsibility to put his room in order, but we must consider if God is fair in His dealings with man. Is a parent fair in charging his children with responsibilities to carry out around the house?