In this verse, we see Jesus intentionally telling us to allow children to approach him. We could be talking about actual children - children who are young in age and immature in their faith. He wants them to approach him. I have an image in my head of an event where an important guest is being received and kids are running up to him and hanging all over him, much to their parents chagrin. The guest is Jesus and he is telling us that it is alright - let them come to him. He should be seen as inviting by even the tiniest believers, not someone who is out of their reach. He wants to get to know these children and for them to know him as well. He is saying that the message he comes to share is just as much for children as it is for us grown adults.
Another way I view it concerns the people who might be considered to be "the least of these" - like Peyton, with all her disabilities and health issues. A child, yes, but someone who some societies would just as soon cast aside and hold back from anything that the rest of society is able to do. In my mind, I see a room full of disabled individuals - people whose intellect is far below what it should be for their age. I see my daughter, just six years old, but in every way like an infant of 3 to 12 months of age, depending on the area you are measuring. I see people full of concern that these individuals are a bother or a nuisance to the honored guest, Jesus. And then I see Jesus telling them to cast aside their concerns. He tells them that all of these people are welcome and that the kingdom is theirs as well.
Then I see the rest of us - his followers. Children of God. I see us in a room with him, full of child-like awe and wonderment, struck by his very presence. We approach him as children of the most high, seeking wisdom and strength. Yet there are others around us who would seek to hold us back, trying to keep us from what Jesus has to offer. I see Jesus admonishing them, asking them to let his children come to him because the kingdom belongs to them as well.