I agree with your reasoning.
Why leave them in bed?
If he did, possibly body fluids will be found there.
The kiddies may have already been placed elsewhere, ready for disposal, after SW's murder was completed.
If in the truck, or other areas, we will soon find out: Blanket fibres, body fluids, DNA etc
As we have read, the estimation of kiddies deaths may be between the 12th and 13th, so possibly he had placed them in the oil tanks, prior to SW coming home: two trips.
I guess GPS, Cameras will prove this later.
I recently, followed a Preliminary Hearing trial: the Cameras, GPS, Phone info presented just prior and during this hearing, was most damaging to the accused.
We spent months speculating with our theories, then 'voila'.
We will find out soon.