Maybe they didn't think whatever Mangum said about how long the kids could stay there was reliable given the shape she was in. I don't know. It doesn't seem all that important, unless I'm missing something on this entire aspect.
I think the main point is that is potentially proves she lied once again to the cops. The only reason they would have sent someone by to check would be if she tried to use the kids as an excuse to "get out" of the situation she found herself in.
"Please officer, give me a break, my kids are home with a sitter, I was supposed to be back by 1:30, I need to get home, please let me go, please".... yada yada...
When that didn't work it was "Oh...by the way, I was raped"...
ta da.... success...
It's just one of those little loose ends that I hate, I guess.