There are various levels to the "search" process and it is not clear from your post exactly what was done.

Just about everywhere, a "pat down" search for weapons is justified on the grounds of safety. This is far less intrusive than what the federal transportation authorities have been doing at airports, which you have probably heard about. It is a quick frisk to see if somebody has a gun, knife, or other obvious weapon. It is not an intrusive search of the person, looking for any item that might possibly have been stolen. Generally, it is to be done by someone of the same sex as the suspect, although that is not necessarily required if no one of the same sex is immediately available. If that was all that was done, I do not see a problem with that, subject to the issue of whether a male did the search or, if not, the manager did it because a male was not immediately available.

Search of bags, such as shopping bags or a backpack is generally okay in most jurisdictions.

A full intrusive search of a person is allowed in some states, but not others. I do not know the rule on this in Wisconsin as I am not licensed to practice there. My general impression is that most stores will not do a full intrusive search of a suspect but will wait for the police to do that.

Generally, interrogation is allowed of a suspect, even a minor suspect, whether or not a parent is present. Continuing interrogation after you arrived and wanted access to your son is another matter.

What I have the most problem with is that you arrived on the scene and the store kept you separated from your son. Although reasonable force to detain a suspect is generally legal just about everywhere, the purpose of detention must generally be to determine whether a crime was committed, to recover merchandise, to identify the suspect, or to hold the suspect for police. Preventing a parent from having access to her son is not generally a legitimate action by a store even when holding a suspect. If they had let you in to see your son and you had tried to take him away before the police arrived, that would have been a different matter, but that did not happen here. Although some judges and other lawyers might disagree with me, there may be grounds here for an action for false imprisonment and/or intentional infliction of emotional distress. I recommend you consult a Wisconsin lawyer to discuss this. I am not saying that you should file a lawsuit or that it would be economically worthwhile to do so, but I do think the store exceeded the bounds of generally authorized conduct in detaining shopifters. I do not say this lightly. Generally, I try to help persons charged with shoplifting at sites ike this, but i rarely find much in the way of problems with the detention process. This is an exception.