It makes no sense to claim they hid the body somewhere and then called the police, thinking to get rid of the body later.
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Why ? Use a litle imagination. Suppose that for whatever reason, one of the parents lost conrol and beat the child to death, hence the blood residue.
In this senario we can see that the crime was not planned. If it were, it would have been executed outdoors, leaving the police with no crime scene, and not places that once held a body, as confirmed by trained dogs. And no blood residue on the floor either.
Now they see how vulnerable they are to capture. They have no vehicle to remove and hide the body somewhere safe. The kid is wraped in a blanket like a sleepng child and carried out, no one who sees suspects anything. Parents carry kids like this all the time.
The body is secreted somewhere, where dogs cannot gain access to alert curious owners. Maybe somewhere in the church ground they were ostensibly fond of visiting.
Its midday, so they go shoping for bleach and other chemicals/detergents. Back at the hotel they bleach and scrub the floor tiles, and bleach the walls and ceilings. As doctors they know this is necessary as the police may search for minute blood samples using search tools. They get their clothes washed at a launderette immediately. Many other jobs are done, and they spend hours thinking of other things that need to be done. They then concoct their story.
Now they need to create an opportunity for the pseudo-knaper to strike. They leave the kids alone, with the door unlocked. Yes they know they will be censured for this, but its getting late and they really cannot think of another plausible occurance to explain the dissapearance of the kid.
They go out for a meal leaving the other two kids alone. They return every 1/2 hour, playing the doting parents publicly for other guests.
They return from their meal, and start crying and screaming. Their dear child is missing. Won't somebody call the police !
Several days later as the heat dies down, they hire a car, and move the corpse to its final resting place. Most likely the sea. Its nearby and all but impossible to search, and the fish will remove all flesh. The bones will never be found.
They can now only be discovered if they incriminate themselves. They believe they can survive any ammount of questioning / psycho - war.
Why Maddie and not the others? - because Maddie is dead. Why not the other two?- because they are alive.