Exactly. The kids were on private property. The store owner(s) can exercise whatever security measures they like. If the kids don't like being asked to open their backpacks, they ought not bring them into the store....or shop elsewhere.
The Constitution limits the powers of Government. Not property owners.
If I’m in a store and some store owner asks to look in my back pack or fanny pack, I’ll tell him to take a hike...he doesn’t have the right to take possession of my goods.
If he thinks I stole something, fine...let him call the police, but there’s no way I’m letting some clerk look at my possessions.
Ed