Question: Why did Jack and Max try to kill Mason's son Sonny? They have their masks on and there's no way that Sonny would have identified them.
Answer: The cover story was that it was a drug deal gone wrong, which led to the murders. The child witnessed them planting evidence and would be able to blow the cover story.





Answer: He survived the first attempt of his family's murder. There's no telling what he saw or heard. As for the masks, the bad guys were taking no chances, security cameras, cell phones and the unexpected eyewitness.
Cell phones? I don't recall seeing a cell phone in the entire movie. It was all landline phones. Cell phones didn't really start to get used until around the mid-90s (a few years later).