At best the bio-mom is an idiot. I don't think that she knew or thought that her son would be beaten, but she had to know a couple of other things that make her judgment piss poor, IMO.
First, she had to know that the house was filthy and unsuitable for anyone, let alone kids, to live in. Graffiti on the walls. In a home where people are living? Really? The whole place smelled of shit and was full of trash and dirty clothes.
Second, there is no way to expect for people to watch 10 kids and not be overwhelmed. Common sense dictates that a single child will usually fare better with a single parent than with 9 other kids and 5 adults. Unless she was homeless or living somewhere drugs were being manufactured I can't for the life of me figure out why she thought that situation was a better place for her kid.
When they found this kid's mom she was staying at another place. Not in a shelter. So why did she leave him behind in a hell hole she no longer wanted to stay in herself? I surely hope it wasn't because she was cum drunk and the kid was in the way of her fix. Whatever the case, she gets 5 years behind the walls to think it over and while she is there I hope her uterus falls out so she can't repeat her ignorance when she returns to society.