Is it legal for a police officer to talk to a minor without the consent of a parent or guardian?

My niece was at the mall her friend stole some jeans and they left. While waiting on the street the police stopped and said they knew they stole and sat them on the curb, her friend admitted to it. But the girls kind of giggled they cuffed them said they were under arrest and put them in the car and then released them. But they said on Tuesday one of the officers present was going to go to her school and speak to her, can they do this. They gave no card with their information anything.