Interesting things I took from this video.
Firstly, she is reaching under and looks over the end of the bed (0:04) as though something has caught her attention before getting off the bed, getting down beside it and then pausing for 4 seconds as though looking for something under the bed. Yes, she appears to start crawling under the bed initially, but then...........
Secondly, she starts calling out for her mum BEFORE she is under the bed completely. She starts calling out at 0:21 but is not completely under until 0:31. That is 10 seconds before being under the bed fully that she is calling out. For those who have had young children, you will know that her cries are fearful and not playful ones and typically, children to not start to call out, if playing around, until they are fully hidden. It is just not the normal behaviour of a child.
Thirdly, I have taken the x8 section of the video and slowed it down to standard time in Premier Pro and reviewed it frame by frame at 200% zoom. The thing I presumed was a leg or hand coming out from the edge of the bed (where the girl went under) was very difficult to make out what it actually was. However, where we believe the girls head is showing at the bottom of the bed intrigues me.
There is a piece of bedding, or clothing, wrapped around the lower left leg of the bed and a corner of the item extends under the bed. This actually moves towards the bed leg over a period of about 3 seconds, but under analysis, I cannot find anything that physically makes contact with the item to explain how it moves in the manner it does. I could surmise that her head makes contact with the material as it comes forward, but then it would not move in the manner seen in the video if it did, and it continued to move for a short period after head started retracting back under the bed.
As for the argument made about the mother knowing where the child was, it would not be difficult to know this from where her cries were coming from.