I had this exact thing happen with my dauchsund the first week. They were in my room because I was still deciding where to keep them. I went outside, my son went into my room, my daugher followed behind him and closed the door. But she locked the daucshund in the room! I come in to find her in the box with 2 of 6 of my cochins dead. I was so mad. Mad at myself, mad at my kids, mad at the dog. Later that day I forgave everyone but myself because ultimately it was my fault for leaving the brooder on the floor. I remmedied this by keeping the brooder up. Currently it is sitting in my empty nursery, on top of the changing table.
I am so sorry. We live and learn.
I am so sorry. We live and learn.