Yes, the police are permitted to detain and investigate suspected wrongdoers.
Whether they had a requirement to notify you of his arrest/detention or not depends on the law and policy. And there is nothing inherent unlawful about being placed in adult holding, though it might violate some law if he was put into a cell with adults. Or, as is the case in some states, within sight and sound of adult prisoners.
If you feel that something untoward was done, you can contact the agency and speak with some and make a complaint, or even consult an attorney to see if there is any grounds at all for a lawsuit ... though, if an attorney says he can look into it for $20,000 up front, you know there's nothing.
More details would be necessary in order to look into the issue further to determine whether all or part of this action was lawful or not.