I have this to add to Mike's fine answer. In context, Jesus is talking prophetically about the persecution his disciples can expect to encounter in the near future. He addresses as well the necessity for his disciples to die to themselves in their service of their master, Jesus, as He sends the twelve out on a preaching mission.
Then Jesus gives His disciples some practical advice concerning their reception by the people to whom they are being sent; namely, the lost sheep of the house of Israel. As commentators have suggested, there is a descending climax in Jesus' words. At the very head of the list is the Father; next is the Son; followed by a prophet; then comes a righteous man; and last but not least a little one.
I suggest the phrase "one of these little ones" refers to all those disciples of Jesus who by dying to themselves find themselves serving others. Each act of service, Jesus assures the twelve, is unto God and has its reward. Just as Jesus came not to be served but to serve and to give His life as a ransom for many (Ma 20:28; Mk 10:45), so too are His disciple to serve and even to endure persecution on His account.
In the sense of serving, then, we all are servants of the most High God and Father: whether Jesus (but only for a time!), or a disciple, or a prophet, or a righteous man, or even a little one. All of God's servants will be rewarded, even a person who in the name of a disciple gives a drink of cold water to one of God's "little ones" (i.e., His servants)--a cup of cold water being the smallest act of courtesy possible, especially when it is given with a smile (rhetorician's translation!).
In conclusion, the "hierarchy" in God's kingdom turns the normal human hierarchy in society upside down, except for the king. Under the King in the kingdom of heaven, however, come not viceroys and presidents and governors and mayors and advisors and lesser bureaucrats, but "little ones" who recognize their littleness before the King, who gladly take their place as mere servants of their King and who distinguish themselves through acts of service to others and thus to God, the rewarder of those who diligently seek Him (He 11:6).