I follow the same logic as the anon above. Mary Lou seems to use her charity as a way to make the kids stay close/do what she wants. Add this with the fact that child protection laws around this time weren't exactly the best there likely wasn't much to stop her from continuously getting more kids even if it lead to overcrowding (which in turn would lead to issues such as disease making beds empty for the next load of children). in fact if none of the deaths could be directly linked to her (or weren't known to the public) then Mary Lou could have been seen as a role model to the community for looking after all those kids. Yes, it's wrong on so many levels but such things did historically occur and were sometimes even carried out by whole institutions (some examples include the Forgotten Australians and a number of orphanages Northern Ireland. A boys school in Florida was closed in 2011 due to a 111 year record of abuse. Again, wrong on so many levels but it did happen and that should not be forgotten)

On a lighter note, it could also be that Credence meant dozens but said hundreds in the way we would say we're "up to our eyeballs in work". It's not 100% accurate but it gives the scope and difficulty level to the person we're talking to.