For the record, I think we should cut Willem some slack.
800 dead children obviously feels like an awful lot, and conditions of their burial are appalling.
But we don't know the actual mortality rate in the institution nor how it actually compares to national average at the time. Over the span of 36 years, that is "just" ~22 deaths per year, while records show the place could house hundreds of children at once.
I looked around and found an article that discusses life conditions and mortality rates in contemporary Ireland:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3053035/
Figure 1 shows infant mortality rates which peak at over 8%, but we don't know how many children actually lived to become adults.
Now, factor in susceptibility to contagious diseases in a place like this and probable tendencies to prefer prayers to medication...