I don't know the case other than the depiction of it in the first Amityville Horror and in brief descriptions.
In familiar mass murders like that, sometimes the killers put the victims back in bed (or otherwise arrange them), although that is incredibly easy for the investigators to see.
Did DeFeo use a silencer?
I'm guessing he shot his parents first, since they were the adults who could fight back and posed the biggest physical threat to him carrying out his plan. (Parents will almost all of the time fight to the death to protect their kids in a situation like this.)
I'm reading the ages of the other child victims as: Dawn (18), Allison (13), Marc (12), and John Matthew (9).
The younger children were probably too scared and might have frozen in their beds, paralyzed in fear and with little choice but to comply and do what they were told to do. Children in general sleep more deeply than adults and less likely to awaken due to noises.
The parents were each shot twice. The children were each shot once, almost like he was in a hurry to finish the heinous act, not wasting time on a second shot. Not to be morbid, but he likely killed them in a particular order of most-to-least physically threatening to him personally, which may not necessarily be by age. At 18 years of age Dawn is basically an adult but not knowing anything about her personality or assertiveness or her relationship with Ronald, I can't really imagine how she would react to a situation like this.
I'm assuming toxicology was performed and the investigators ruled out that any of the victims had been sedated in advance (say, at dinner)?
Haunting case.