This is complete speculation on my part, so take it as you wish. She doesn't strike me as a very complex or thoughtful person. She acts out of illogical anger, her actions are not very well thought out, she seems impulsive. I don't think she made a calculated plan to kill Lucas, I think it was the result of some impulsive action or accidental. Whether impulsive or accidental she would have freaked out, as most anyone would with a dead child on their hands, she would want to get rid of Lucas as quickly as possible, get as much distance between her and Lucas as possible without drawing any attention. If my theory is correct, I think she probably left him somewhere within the city or just on the outskirts. I wouldn't be surprised if he's found in a ditch or culvert covered up with leaves and such or maybe where the body of a child would blend like a junk yard.
It's hard to get a good read on her from the outside. She seems like a study in contradictions, at least to me as an outsider with no knowledge of her actual personality.
