Can a twelve year old be a secret shopper?
Labor Laws - no you cannot be a secret or mystery shopper
yes, they can be left alone if the twelve year old is be responsible, if they are not responsible then hire a babysitter for not only the 2 year old but also the twelve year old so that you can embarrass that twelve year old. (inviting over their friends while having the babysitter might just do the trick) and make sure the babysitter is very unattractive and old.
It depends on the 12 year old in question. At 12 years old tampons aren't recommended as even if they're mature enough to understand risks and use tampons correctly there is still a higher risk of developing Toxic Shock Syndrome. Pads are recommended at first but if they want to use internal options then it's recommended to consider safer options like menstrual cups - younger women would need to consider smaller brands like Ladycup.