No, your child is not going to become president of the United States, but supporting those goals teaches them that it’s a good thing to aim high. Becky Kidbeater is probably not going to become an astronaut, but a lifetime of following that path gives her a much higher likelihood of landing a pretty damn good job along the way. As an adult, it’s easy to look back and recognize that you aim for the moon so you can land on mountaintops. That’s not really something you can teach; it’s something you learn through experience.
Of course, you could always attempt to explain this to a child. You just have to be prepared to handle their look of dumbfuck bewilderment, because they can’t grasp complex ideas like that yet. Or you could wait until they’re teens and watch them either give up their dreams because of your honesty or roll their eyes with that universal expression that says, “You are impossibly stupid. Just because younever made anything of yourself doesn’t mean you have to shit on my dreams!”
You will lie, because it will make them a better person. Doing anything else makes you a dream-crushing nutsack.