In this verse, Jesus focuses on the example of children. Children show something of what he expects from believers.1
In the past, Christians understood this to be a child's humility, innocence, or some other characteristic common to children.
Instead of looking for qualities and characteristics, however, Jesus is focusing on the status and position of children. Children had no status in Jewish society; everything they had was given to them. They are completely dependent on others to care for them and help them. They don’t bring anything to adults in exchange for care.
Therefore, to receive the kingdom as a child is to welcome it on the terms on which it is offered. No one has the right to possess the kingdom of God because they are worthy in and of themselves; it is a gift of divine grace.2
To receive the kingdom of God is to accept the rule and authority of God, to become God's willing subject. The kingdom of God must be received as a gift from God in a simple, trusting faith just as a child receives a gift from a loved one without any sense of having earned it.3
15 Truly, I say to you, whoever does not receive the kingdom of God like a child shall not enter it.”