I think it is very relevant to the situation that neither parent saw fit to admit to turning the children's bedroom light on despite being unable to see the children clearly. Why was that?
I can understand why Gerry didn't; he was probably focused on getting back to his dinner and didn't want to wake the children. In Kate's case she sees;
A bedroom door open much further than they had left it.
She, unlike Gerry, doesn't wonder if Madeleine did it. Her only speculation seems to be about Matthew. Rather than checking the children she decides simply to return the door to it's normal position. It slams and she opens it again only to see the whooshing curtains, an open window and raised shutters.
She then enters the room and peers around in the dark to see if the children are there. She doesn't turn the light on because she doesn't want to wake them.
Firstly why was her first thought to check that the children were there? There are signs of a break-in and the person who did it could be anywhere; even in the bedroom with your babies. This is not a situation where you creep into a dark room, you turn on the light!