No violation of Constitutional rights occurred whatsoever. It is surprising how little we actually know of what "rights" actually are.
Sorry, I would differ with you. A person being asked to have their personal affects inspected in the store is being detained for shoplifting. This is where it gets a little odd...if the boys had been detained for saying No and the police called...the store would need to present probable cause.
There is a distinct reason for the retail shoplifting protocol.
http://www.crimedoctor.com/shoplifting2.htm
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.