If this diagram is drawn correctly it would be easy to go from the kids bedroom across to the parents room without being seen from the patio.

I agree, risky but just about possible, with the patio door opening at its western side.
I have always wondered if the Abductor could have come in via the parents bedroom patio door tho, cos that was hidden behind a bush from where the Tapas group were sitting. I tried to awaken interest in it a couple of years ago, but IIRC, not a soul responded. Good to see that you, Rob, could see the possibilities.
No forensics were done on the parents patio windows. I wonder why? Maybe it was locked or maybe the photo which Carana just showed was as it was at the time. However, the parents left in the daylight. Would they have closed the shutters in the daylight?
As John pointed out about that time on forum, most of the steps up were not illuminated by the street lamp opposite, anyone ascending was in shadow part of the way .... so a pretty well concealed way in. Although there would have been some illumination from the distant street lamp by the little car parking area opposite the Tapas Reception.
I still think the front door is the obvious and easiest way in and out. Hidden away in a recess that is not passed by anyone .... and dark. With the wall to the pathway, it would be easy to "bob down" to hide oneself completely had anyone come across block 5 parking area along the usual track.