Any intruder inside apartment 5A, would have no need to climb out of the children's bedroom window, there was no need for any would be kidnapper to force open the window shutter, or to force the window to enter the premises, because according to the McCanns they left the sliding patio door on the road side of apartment 5À unlocked. Not only that, but a suspect would know that the patio door of apartment 5A might be unlocked if the metal shutter on the patio door was raised up. However, the door itself could still be locked. Alternatively, if the shutter is fully down, the door beyond the shutter could be unlocked. The mechanism for raising and lowering the metal shutter to the patio door, and for locking, or unlocking the patio door itself are fitted on the inside wall and or door.
I don't believe that the McCanns left the metal shutter of the patio door of apartment 5A fully raised, and the patio door itself unlocked. At best the shutter was lowered down when they left, and the patio door could have been left unlocked, but you wouldn't be able to enter the apartment via the unlocked patio door, because your ability to get beyond the lowered metal shutter which can only be raised and lowered by operating the raise and lowering mechanism fitted inside beyond the locked off metal shutter, and the unlocked patio door...