Kit I agree, we just had that conversation in our house. She knew there was something wrong, she couldn't understand why she felt different towards one of her children and hence the google search. I don't believe she knew the full extent of how this was going to end, I think she thought the bruises would heal and she hoped to find a way to make them go away and send him back to nursery.
She couldn't have been all there in the head to just dump his body. A mother wants warmth and love for their children.
However like you I do not condone it for a minute. She had time to get him help - he was already sick prior to the beatings. She had our city looking for him, folk are furious round here. Total fury.