Did you buy a ticket for a minor? There's no option for a minor's ticket.

No need, age not exceeding 14, height not over 1.2 meters, must have a guardian.
No need, but you need to make an advance reservation, advance reservation is very important…
No need, the Forbidden City does not require admission tickets for children under 14 years old and those under 1.2 meters in height; they must be accompanied by a guardian during the visit.
Free, but requires advance official reservation, and must be accompanied by a guardian.