Vatican Museum - Proof of Age for young children

I am getting ready to buy our tickets online the Vatican Museum. Our daughter will turn 6 this Sept. She is tall for her age. Vatican Museum says children under 6 are free. Would I need to bring our passports with us? Do they generally ask us? Or should we just go ahead and get her a ticket?

It is advisable to carry our passports or leave them in a safe at the hotel?

Take a photo of the passport page.

I wouldn't pay for a ticket if they say it's free. ☺