When I was a kid, my parents kept meat birds and one morning we woke to find ALL BUT ONE slaughtered. It was a neighbors dog, and it tore through the coop and just killed them. Ate none, took none, just sport. I don't think it was a dog, hon. Likely something wild because I think dogs do it for fun. The lone survivor was upgraded from "meat bird" to "pet" and was named Greenie. (It was white, no idea where that name came from)
I feel for your 6 yr old, about the age I was when this happened. But do start over. Reinforce your enclosure and start again or else it will leave a bitter taste in your mouth forever.