This attack was foreseeable if the parents knew the warning signs. First, this was a male dog. Male dogs account for 75% of dog attacks. (See Why Dogs Bite.)
Second, it was a pit bull. According to the Clifton study, pit bulls, Rottweilers, Presa Canarios and their mixes are responsible for 74% of attacks that were included in the study, 68% of the attacks upon children, 82% of the attacks upon adults, 65% of the deaths, and 68% of the maimings. (Clifton, Dog attack deaths and maimings, U.S. & Canada, September 1982 to November 13, 2006;
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Third, the dog was kept chained. Chaining results in aggression (26-28% of dogs involved in fatal attacks were chained at the time). Journal of the American Veterinary Medical Association: Dog Bite Related Fatalities from 1979 through 1988 by J. Sacks. R. W. Sattin, & S. E. Bonzo. Volume 262, pages 1489-1492.