My perception is that AD was doing her best, and money wasn't exactly flowing abundantly into her bank account. I'm sure that if it was even remotely workable, she would have paid (or traded) for a babysitter for after-school and other times when she couldn't be at home. Apparently she trusted SE as a babysitter for J, and moved into SE's home with J.
These things break my heart because sometimes it might be for lack of adequate money that things like this are more likely to happen and people cannot be protected from harm. Somewhat different situation, but even Elizabeth Smart (family of affluence) was kidnapped from her own home. Something horrible like this could happen to anyone, IMO.
I wish I knew why such things happen, and why some (and so many!) people do these unconscionable things.