My Mastiff killed my Min Pin and I don't know what to do, I think it was an accident but I cant be sure...?

I have had my mastiff for 4 years and I thought she was the sweetest dog. I have 2 children under the age of 2 and she is absolutely wonderful with them... when strangers come to the house, she will bark, but once we let them in she runs to her cage and hangs out there until they leave. We got a min pin / poodle mix a year ago, and they have had typical sibling spats which 2 of ended with the min pin bleeding, but nothing life threatening, which considering their size difference (140lbs vs 15lbs) I figured the Mastiff just kinda "forgot" she was bigger... Normally they get along very well they eat together, sleep together, no issues. Last night we were woken up by our neighbors who heard the dogs, but it was too late, my little dog was dead... I don't know what to do, she wasn't mauled, there was barley anY blood, and her neck was snapped.... But can I continue to trust her with my children? Like I said, she has never shown any type of aggression towards people, and she has only ever barked at other dogs, aside from the 2 incidences with "her sister" Please help, I don't know what to do...