Jesus welcomed the children. This attitude is harmonious with His attitude toward all the humble, dependent, needy, trusting, and vulnerable people who came to Him. Furthermore children coming to Him symbolized people with the characteristics of children coming to Him. Jesus did not want to discourage anyone like them from coming to Him. He did not say the kingdom belonged to children but to people who are similar to children. Children provided an excellent object lesson that Jesus used to illustrate the qualities necessary for entering and serving in the kingdom. [source][source][source]
The difference between this lesson and the one in chapter18 is that there the focus was on the childlike quality of humility that is so important in a disciple. Here Jesus broadened the lesson to include other childlike characteristics all of which are important. [source][source][source]