Am i wrong..? mother in law issues?

The other night my mother in law took my oldest son over night. Well at 10pm (hes 6 mind u) I found out my son wasn't at home in bed with that he was a bonefire with his uncle and aunt 24 and 19. needless to say they had both been drinking and i was upset to find out he wasnt home with his granparents where I thought he was. I sent my husband to get him and when he got there my son had a busted nose... he had run into a picnic table.we contacted his grandmother and she was angery with us for going to get him.. that she felt upset that we didnt respect her dision.we tryed to explain we believed he was in her care home in bed....are we in the wrong ?