I simply take it as a description of reality. As a child, you are nothing yet, but you have the potential to become almost anything. As you get older, you trade in that potential for something specific. It's up to you to choose. You can trade all your potential for one thing only or you can divide it up to become multiple things. The right balance is debatable. The real danger lies in taking it to the limit. If you try to become everything, you divide your potential up into infinitesimal pieces and end up remaining nothing at all, a child.

In any case, there is some real wisdom in Peter Pan, but it was completely lost on me as a child.