Jesus said, “Let the little children come to me, and do not hinder them, for the kingdom of heaven belongs to such as these.” Matthew 19:14
And (Jesus) said, “I tell you the truth, unless you change and become like little children, you will never enter the kingdom of heaven. Therefore, whoever humbles himself like this child is the greatest in the kingdom of heaven. And whoever welcomes a little child like this in my name welcomes me.” Matthew 18:3-5
First off, all our hats should be off to Mothers and Fathers - the welcomers of children. According to this last verse, when they (or anyone) welcomes a child in Jesus name, they welcome Jesus himself. That itself is wild. When would they be welcomed? Well, I suspect when a mother finds she is pregnant. Either that or when a child runs back to the house for dinner. I suppose either way would work. Anyhow, that is a side-thought. But what a thought!
I often watch my niece and nephew on Mondays. They’re two years old, and it’s such fun to see how they’ve grown each week. I’ve made a few observations since. It appears that children are earnest. They do only what they want to do, unless taught to obey. They are without guile. They need attention and affirmation like crazy. They need boundaries, love, directions, fun, lessons, discipline, comfort. Sometimes they take toys away from each other. Other times they give kisses when the other is crying. Being earnest, they don’t live as they ought to. They live as they do. There is no “ought” involved.
The kingdom of heaven belongs to those that are “such as children.” Child-like folk. Earnest people. Not good people. Children don’t seem inherently good. They seem inherently selfish, and inherently needy for love (And yes, inherently adorable). They earnestly desire their parent’s approval. The kingdom of heaven, as far as I can tell, belongs to those who are parented by the Lord.
So, what’s the use of growing up? What’s the use of coming of age, learning to be an adult, if Jesus tells his disciples that unless they change and become like a child, they will never enter the kingdom of heaven? These are the disciples! They are in danger of not entering the kingdom?! I get very sad and confused by this until I remember that they were prodding Jesus to say which of them was the best. That’s what started this whole thing.
I don’t get how a child is ”humble” and still outrageously selfish. But then again, an adult is full of “ought-to’s” and responsibilities, and still pretty darn selfish. I guess a child knows he is small, knows that counters are too high for him. He still wants to know he is the center of his parant’s heart, but he cannot be the greatest. Mom and Dad are much greater.
So then, we can still grow up. We get the green light to become adults. It looks like what Jesus was getting at was that he wants us to become earnest people. We are to remember God’s greatness, and how high the counters really are.

That’s a good question! Why does Jesus tell us to become like children, and then Paul says we should put childish thoughts behind us?
I think that though children can be selfish, as you said, they don’t put on airs and get filled with pride of their own self-importance. It’s different. When adults are selfish, they think they already know everything, and it’s harder to teach them the ways of God (sometimes). Children, I think, are often more eager to learn. That’s probably what Jesus was getting at….