God's measures greatness differently than the world. In the world, the great people are the popular people. The great people are those with a high status and much authority. And it is the old people who are honored. But children and servants are not reckoned great.
Notice that he associates children with servants. In that culture children were expected to obey and serve their parents not unlike servants. We see this for example in Paul's writings: "What I am saying is that as long as the heir is a child, he is no different from a slave, although he owns the whole estate. He is subject to guardians and trustees until the time set by his father."Gal 4:1,2 So also does Jesus illustrate this in the Parable of the Prodigal Son in which the older son objected to the younger's treatment saying to his father "Look! All these years I've been slaving for you and never disobeyed your orders."Luke 15:29
Children and slaves would perform similar tasks. Many today have deviated from this concept of raising children. And it's important to understand this, else we would interpret greatest to be simply a function of being immature and acting childishly. In Jesus' culture "childlike" is not the same concept as "childish" today. Christians should not be childish. "When I was a child, I talked like a child, I thought like a child, I reasoned like a child. When I became a man, I put childish ways behind me."1Cor 13:11 Rather Christians should strive to be servants. And indeed, understanding this, Christians like to be called servants. But on the other hand Christians often don't like to be treated like servants.
Jesus is the ultimate example of a servant. "For even the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many." Mark 10:45 5 And Christians are called to be servants of Christ. "He died for all, that those who live should no longer live for themselves but for him who died for them and was raised again." 2Cor 5:15 The Christian life is not like signing a free fire-insurance policy and then going on your merry way. Believing in Christ incorporates submitting to His Lordship as well. Christians should not spend their lives running from here to there simply to find someone to meet their needs. Rather we should consider how we can meet other people's needs. Are you at a church that doesn't seem to meet your needs? Well then maybe you could meet theirs instead.