My guess is that the children were killed in one of three ways. Poisoned, strangled or suffocated. I would consider any of these options to be torture. It breaks my heart. If they were killed by being shot or stabbed it would have caused too much noise.
I agree.
Unless there really was a gunshot. (i.e. Maybe they realized Tylee was "just" unconscious when brought to CD's backyard, so AC or CD decided a shot to her head was the best way to make sure she was dead.)