Before you compare
Two things to know
Count every device
An account follows your child across devices: the same account on a phone at home and a tablet at grandma's counts all of it. So compare the account's total against the SUM of all devices, not any single one. Missing a device makes the gap look smaller than it is, never bigger.
A gap is a conversation, not a verdict
Shared devices, siblings, and measurement quirks can create small gaps. A large one usually means another account exists. Either way, the productive move is a calm question. A crackdown teaches the next account to hide better.