There's a lot going on in that little paragraph. But I'd like to highlight something that is often missed. When Jesus sets the child by his side this is not first of all a metphor for something else. Receiving a child is not an allegory pointing to something “deeper” or more spiritual. It’s not simply an “object lesson.” Jesus means first of all that true greatness in his kingdom is about receiving little children. You want to be great? Then welcome little children in my name.
By placing the child by his side, Jesus is identifying himself with the child. You receive this child, you receive me. This is similar to Matt. 25 where Jesus says that "when you do something for the least of these my disciples you do it for me."
Children have a position of honor in Christ’s kingdom, and adults better get with Jesus’ program. This is also evident from Paul's discussion in 1 Cor. 12 of the seemingly weaker members of the body. They deserve special honor. And remember all of those times in the Gospels were the disciples are rebuked by Jesus for not paying attention to babies and little children. Suffer the little children to come to me, and so on.
There's another interesting thing about Jesus' words. The language of "welcoming" repeated 4 times here by Luke was virtually a technical term for hospitality in the original language. Receiving/welcoming a little child means showing him hospitality, honoring him as a guest at your table.
In his commentary on Luke Joel Green notes that Jesus action and words
"undermines everything that the Roman world would hve taken for granted regarding questions of status and social relations. 'To welcome' people would be to extend to them the honor of hospitality, to regard them as guests (cf. 7:44-46), but one would only welcome a social equal or one whose honor was above one's own. Children, who's place of social residence was defined at the bottome of the ladder or esteem, might be called upon to perform acts of hospitality (e.g., washing the feet of a guest), but normally they would not themselves be receipients of honorable behavior. Jesus turns the social pyramid upside down, undermining the very conventions that led the disciples to deliberate over relative greatness. . ." (p. 391-2).Jesus says, make children honored guests at your table. If the church is a family, and the Lord's Table is the ritual center of the family, then the children ought to be there.
In the Roman and Helennistic culture of Jesus’ day, children were not honored, they were little higher than slaves on the social scale. Jesus turns the social pyramid upside down. I think the church still has a lot of learning to do about the place of children in our gatherings, especially at the Lord's Supper.