I wouldn't be super shocked if Al didn't know or even suspect that she was abusing his kids. It actually happens all the time, and when a kid is murdered by a step-parent, usually the biological parent is pretty shocked. Most of the times, when stuff like this happens, we can't really see it coming. Sure, there are many, many, many cases of pretty obvious and constant abuse that end up in a child murdered by someone that should've cared for them, but there are also several cases where a step-parent is indeed mean spirited torwards a child, but doesn't physically hurt the child and, all of a sudden, just snaps and kills the child. So, this is why I usually don't jump right away on the biological parent's throats because, in many cases, they're just as shocked as we are. And, in Gannon's case, I sort of believe that she was probably really mean to them, but didn't necessarily hurt them physically, because Landen said "I trusted her". I have a hard time believing that Gannon wouldn't have spilled the beans to his mom, to whom he talked everyday, if she was hurting them.
I don't understand why she chose to tell that ridiculous story about Gannon staying home because he was sick and then leaving to go to a friend's house. It makes no sense. She leaves with him, when she comes back, the kid is nowhere to be found. Why wouldn't she just say "I dropped him off at his friend's house on the way back home", or "I let him stay at the park waiting for his friends and I came back home"? Why bring him back home, kill him in his own bed and having to clean all of that blood when she could've just done it in a wooded area and leave him there? Makes no sense. I always thought she had killed them and hid the body before coming back home, but all that blood in his bed makes me think otherwise.
