Everything is in the syntax ... Fiona might have thought it strange that the children slept through the police arrival ... but she did so only after ascertaining and forming the professional opinion that the twins were "fine" and "sleeping" while under continued close observation and close monitoring.
She of course had the advantage of being there to enable her to form her opinion which I think may well outweigh your speculation.
You have no evidence that Fiona ascertained anything, formed any [rofessional opinion or that anyone continued to monitor and observe the twins. Those are all assumptions.
Fiona tells us that 'fairly soon' Emma Knight arrived and from then on until the GNR arrived it was Kate, Emma and herself in the apartment with Gerry and David 'coming and going'.
Emma mentions no concerns about the twins and no monitoring or observation of them;
I went to the McCann's apartment, entered by the patio doors and introduced myself to Kate and Mrs Payne. I entered the apartment living room and Kate and Mrs Payne stayed in the main bedroom, from where I could hear them both crying.
http://www.mccannpjfiles.co.uk/PJ/EMMA-LOUISE.htmWhen Silvia Batista and the GNR arrived the twins were in the bedroom with the door closed.;
She also recalls entering in the room where Madeleine should be sleeping and remembers now that the door was closed. The room was dark. The blinds were down, some light entering through their holes. The windows were closed and the curtains slightly open. Gerry, who followed her and the elements of the GNR, said he did close the window because of the babies sleeping in the room, a fact she confirms.
http://www.mccannpjfiles.co.uk/PJ/SILVIA_BATISTA.htm