There are two key thoughts here. First, Jesus is talking about actual children. His rebuke of the disciples when they didn't welcome children, but pushed them away, is clearly an example of that.
But, there is a deeper issue that will dominate the rest of the chapter. Because children in Jesus' culture were powerless, without rights, and vulnerable, they became the symbol for disciples who are powerless, without rights, and vulnerable. On the surface, they may not have anything to offer the community. But Kingdom thinking is upside down to "the way men think." To be on the side of God means to be on the side of welcoming, accepting, and receiving "little ones" whether they are actual children or simply disciples who are little in the sight of the world. Because Jesus adamantly rejects the normal channels to fame and fortune and instead walks the way of the cross—the punishment reserved for the powerless, those without rights, and those who are vulnerable—his disciples should welcome and care for those like their Savior!