There are schools on military bases all over the world. Here's one at Joint Base Andrews: https://maps.app.goo.gl/iMm4QSZJYAaLLSLh9

Are the children in that school a legitimate military target? Is putting that school on Joint Base Andrews "recklessly stupid?"

Why is it perfectly fine for the United States to do this but "recklessly stupid" for Iran to do it?

Because the US military publishes maps of the base. Anyone who bombs a school on a US military base is doing it intentionally. China could probably call up the DoD and ask for maps of every base, and they would get it.

If your force your enemy to decide what is and isn't a civilian target, you are the deranged one.