Why not? I would say that it's very important to understand that even middle class Americans are quite well off when compared to the vast majority of the rest of the world.
A child doesn't ask if their parents if they're rich because they looked up the GDP of various nations. They ask their parents if they're rich because they see differences between the things they have access to and the things their peers have access to.
Even a child knows that when they have a bigger house, receive better gifts, visit more places - that they are different from their peers and even families on TV. They deserve to know that most everyone they actually interact with will consider them "rich" and treat them accordingly. They also deserve to know that being "rich" is truly not the norm and that acting like everybody in the USA has the exact same opportunities as them is going to leave them out of touch with reality.
It's not theoretical. It's factual.