Parents, did I over-react?

Yesterday, I came home from being at my mom’s house and saw that my neighbor’s door was slightly ajar. I could hear young kids inside but I went into my apartment. After about ten minutes, I peeked outside my door and saw the door was still ajar and I could still hear the kids. I knocked on the door to let their parents know that the door was ajar, thinking the door hadn’t latched when they closed it or that maybe the kids had opened it while playing. Two little girls, no older than six or seven opened the door. I asked them if their parents were home and they didn’t answer me, they just stood there giggling. When I asked again, two adults came into the hallway and said they were the girls’ parents and went into the apartment. I told them I was just making sure they knew the door was open. They told me they did and I went back to my apartment.

Was I wrong to try and let the adults know the door was open? It was very obvious that there were two young children in the apartment and when the girls opened the door themselves, I was glad that I had knocked on the door and not someone less harmless.

Am I over-reacting? Parents, would you be okay with leaving young children alone in an unlocked apartment for any length of time?