When the murderer is locked in their cell, they enter the room through a barred door. As the door closes, they're viewed through an observation window - in a solid door - which is then closed.
The newspaper Helen and Jenny look at has a period at the end of the headline. Newspapers headlines don't have periods.
When Helen opens the door to Jenny the key is in the lock on the outside.
The kids attend a school yet never notice the surveillance camera.