In the Bible, there are a few, almost incidental, interactions between Jesus and children. Apparently, children were among the people constantly crowding around Him. Common to each of these stories is a strong correction from Jesus to the adults trying to manage them. Presumably some of the chastened were parents. The children in each of these stories seem to be (as we may well remember our own childhoods) adventurous.
They were probably not hovering near Jesus to learn. It is unlikely these were little theological protégés. They were not there to be healed or because they needed something from Him (except for His blessing). At least we have no reason to believe that, because no such reason is given. They were not there to lure Him into a religious trap. It takes a while for human beings to become Pharisees. So what was their motivation to be around Him? What was it about Jesus that intimidated His rivals, exposed His friends, but attracted children?
Likely, they wanted what all kids want: to be close to those who love them.
So, it is that childhood is not a missionless season of our lives. Instead, it is a foundational time when we are called to be exactly what it is easiest to be—a child. Children already belong to the kingdom, representing it, even bearing witness to it in some unconscious way.
When Jesus says, "Let the children come," He might also be saying, "Let them explore the world as a place of adventure and not a place of restriction." In turn, when He says, "You must become like one of these little ones to enter the kingdom," He is saying, among other things, that what they are is enough. It may be the only time in our lives that what we are is all we need to be.
The writers of the New Testament will go on to understand childhood as an essential and foundational theological construct. We are all meant to know ourselves perpetually as children of God. For that is what we are. Conversely, not as adults of God. It is this pure state before our transition into adulthood that we return to in our foundational relationship with God. No matter how old we are, He remains our Father, and we remain His child.