The disciples wanted a ranking. Who is greatest? Who gets the best seat when the kingdom comes? Jesus does not give them a list. He calls a child over and puts the child in the middle. In a world where children counted for little, that was a shock.
He says three things about children. Adults must become like them to enter the kingdom: dependent, trusting, unconcerned with rank. Welcoming a child is welcoming Jesus himself. And harming a child, or leading them away from faith, is among the worst things anyone can do.
That last warning should make every parish and family careful. Children must be safe in our churches and homes, and their faith must be protected and nourished. Jesus says their angels see the face of the Father. Whatever we do to the little ones, heaven is watching.