Children need to be aware and experience how beautiful they are on the inside, so they want to stay like this and show more of it. This awareness training helps children to get to know themselves. It gives them the desire to choose for the beautiful sides of their character.
When I first learned that one needs to be like children in order to get to heaven, I thought it a figure of speech. Many years and three daughters later, I understand that this is as true as saying there is a sun up in the sky.