Was it an accident, totally unpredictable? Despite various arguments to the contrary, rottweilers are perceived as potentially dangerous in many countries. Anyone keeping them cannot reasonably claim that they were unaware that the dogs could behave undesirably. And there have been cases in other countries where landowners have been held responsible for injuries sustained by children who managed to break in - the argument being that the onus on keeping intruders out becomes greater as the possible consequences of such an intrusion increase. Having potentially dangerous dogs roaming freely within the enclosure increases the responsibility of the landowner in ensuring that children cannot physically get in. These kids don't seem to have had much difficulty climbing over the fence, and they were clearly totally unaware of the danger that was inside.

No, I don't believe the word "accident" is appropriate. "Misadventure" might be better, and that doesn't exclude the possibility of blame.