The same thing that stops people from doing that in a regular store? Mostly good morals backed up with cameras and the occasional security guard.

Theft rates can get pretty high, look at the Tide theft phenomenon.

I don't know how they designed it, but if they use anything like a phone-unlocked turnstile or revolving door to let people in, they might just count every person that comes in until the gate closes again as being attached to the account of the person that opened the gate. So when any of them leave with something, that's a charge to you.

I could see it being a case of scan, pass the phone back to child/whoever's with you (or scan it their side for them), then they're all registered on that account. A bit like plane digital boarding cards. That does remove a potential fraud signal though so maybe they have a better idea.

Everyone entering the store has to be scanned in. A parent could bring a child in, but they are scanned in on the parent's account. Similar to how at some offices, visitors are scanned in using the employee's badge. At least, that is my understanding of how it works.