But the whole problem is there are no cases where a killer had to report a victim kidnapped for ransom to cover the death (suggesting a runaway is a different situation for a variety of reasons). Child killers of any kind (stranger, friend, or family) don't use that as a cover up with any amount of frequency.
It's just as much something that people don't even think of, because all it does is make police look really hard for the child shortly after the crime, and limit the timeline/possible perpetrators. That's why they usually claim the child disappeared and they found the window open, and usually suggest a crazy child predator. They can keep that story going for a long time. With a ransom demand, the cops know something is up with the story as soon as it's not followed up with appropriately. Plus, you obviously need to be somewhat wealthy to make it plausible.
The closest I can think of is Casey Anthony, but she never intended to go to police with that story and didn't frame it as a kidnap for ransom because that would have required her to report it much sooner than she did.
There really aren't any comparable situations I can think of that you could use to say "people who fake kidnappings for ransom don't call 911 until they dispose of the body." However, there are many situations where family members who kill a child report the child missing (suggesting a kidnapping), usually after the child has been missing a while, and yes, they have always disposed of the body.
Because of the timing with Christmas and the flight, if the Ramseys were involved, their timeline ended up being unusually limited. If they hadn't called 911, and instead delayed the kidnapping claim, they would have had to explain the suspicious change in plans. They would have probably needed to wait at least one more night, canceling their trip and explaining to Burke where she had gone, and inventing a whole day of JonBenet's activities, in order to say they weren't awake when the person entered. Yes, calling 911 early was not logical taken alone, but running out of time would explain it.
Criminals take risks, but so does anyone else, and any choice in this situation involves some risk. Waiting wouldn't have necessarily put them in a better position, particularly if you imagine the stress of living with it for another day - I believe whatever happened, they wanted it over with, if they were involved.