It would not be unusual for the parents to have been asleep all night. It's a perfectly feasible alibi for a crime which could be committed in less than a couple of minutes in the middle of the night - whilst the parents were fast asleep in their own bedroom.
No you keep ignoring the key issue. There would have been no independent witness verification of them, or their movements on or around the crime scene at the time of the crime happening.
This would instantly put them in the frame with investigating police.
After all in most child disappearances statistically speaking family members or friends are more likely to have carried out such crimes than random strangers.
So under your scenario the only evidence to support the parents would be their own statements which could not be verified by third party witnesses.
It would throw up a huge red flag right away with investigators.
Isn't this what had happened elsewhere in this area in the previous 3 years - where offenders got into apartments during the night and sexually assaulted children.
Is there a source for these attacks?