I've been watching kids programming lately, and I hear this theme over and over again. It's supposed to be a self-esteem builder but I think at the root of it's a highly destructive thing to tell kids that sets them up for disappointment.
It's simply a lie.
A kid can't be or do anything. I wanted to be an Astronaut when I was a kid but a congenital heart condition kept me from doing so. I knew it would so I didn't try. But very few people get to be Astronaut or President.
So I know that it's sort of a mindless platitude that we tell children, but I am not sure that we should be saying things that we don't really mean to kids like that. They should know that they will be suited to certain things and not others and that it's important to find out what they are good at and will enjoy enough to apply themselves to. I am not saying we should do the Jewish Mother thing and tell every kid they should be a Doctor or a Lawyer, we shouldn't necessarily tell kids what they HAVE to be, but I don't think we should say, 'You can BE anything.', we should say, 'There are so many jobs to choose from you should see which ones seem the most interesting to you.'