Yes, it was an accident, not negligence. I cannot stand this society which wants to sue for every single unavoidable mishap. Stuff happens.
Securing against accidents by anticipating activity-—removing hazards is reasonable. Monitoring those placed in your charge is reasonable. A head count periodically would make sense, as some are adept escape artists. Illness could occur, should be watched for in a group setting, and would require evaluation more often than a 6 hour interval. If this were a human child daycare, no one would say accidents happen.
Leaving a child to play alone in a sun baked yard with containers that could result in entrapment is a close analogy to this incident. For whatever reason, no one noticing someone was missing for 6 hours is a problem, and it isn’t simply an accident.