I live in MN and would to place an audio recording device on my son, is this illegal in my state?

I am not sure if this was placed in the correct Legal Topic, as I did not see privacy listed. I am not sure if my son is telling the truth with situations that occur between him and the other children at the playground. Personally I do not believe him as I have caught him lying. Since I am not recording phone conversations, am I able to place a hidden sound recorder on him in order to record the happenings that occur when he is playing outside?