I think it is highly probable that something like this happened.
But if there was a struggle of some sort, wouldnt the kids have heard it?
I don't know if I'm right, but it seems as if all three children were in the house with Michael (and maybe the cook was too?).
All the children must have been sleeping when Michael got back, but wouldnt they have heard the noises (thumping, screaming, yelling, furniture falling, talking...) and woke up because of them?
All the kids may be heavy sleepers...but at least on of them is bound to sleep light.
Maybe one of the kids did hear something, and they were told to go back to bed by someone who was in on the whole thing?
I think that if any of the children heard any suspicious noises coming from thier fathers room, they would go in and check, even if they had been told not to.
Kids can tell if there is something wrong.