I didn't know Facebook had an AI nudity filter, I thought it was all based on reported images. They softened their stance on some things, especially after censoring a Pulizter-prize winning photo from Vietnam, and eventually changed their policy on breastfeeding too https://www.facebook.com/help/340974655932193/

I'm sure they don't have a legal exception for underage photos, but those laws do vary from state to state.