It does not seem that JW would have known in advance of that night, that the child was going to be there that night so he might let him in. It was a last minute thing that she brought the child to FR.Too risky. He would have identified Jake. Although Jake was masked, a kid letting him in and following him around while he commits murder wouldn't work. Some how Jake by-passed the sleeping kid I believe.
I also think Jake used to live in that trailer part time with HR and would have had the key.
I am not sure he even knew that the child was there that night when he went to commit the murders, before he gained access.