small children missing for 6 weeks and no cadaver dogs sounds really out there for me unless they are absolutely sure they are not in that area
I think LE must be absolutely sure Lilly and Jack are not deceased and in that area.
As to why LE could feel they can be absolutely sure Lilly and Jack weren't deceased on the property or in the woods, they have been investigating and have collected information along the way, leading to asking DM to do a polygraph test and him agreeing and them giving him one.
We don't know how LE interpreted the results of DM's test, or whether or how many others they gave polygraphs to (IIRC, there was an article linked here a few days ago that said they were considering giving them to up to 54 people, or did I dream that?? Apologies if so).
So whether giving polygraphs is just a standard weed-out tool because they simply need to know more and are looking out further into the parents' social circles, neighbors, known criminals in the area to see if anything comes up related to the children, or they think someone may be lying about the circumstances of their disappearance, it's hard to tell.
IMO & MOO:
Maybe they saw something about the property and terrain or access to the woods or tire tracks that pointed away from that area or seemed to be a very unlikely route to have been taken by young children playing, and suspected an abduction was more likely by someone driving by or driving up and grabbing them and speeding away.
Maybe they saw something or heard something from the parents when they arrived at the house that day that seemed off about their descriptions of what they said they were doing when they vanished and/or the timing between then and calling 911, the condition of the home, their demeanor or lucidity, their nervousness or lack thereof, meaningful looks or looks of guilt or fear between the parents, that LE assessed could have gone beyond just feeling bad they were in bed and didn't know where their kids had wandered off to.
And perhaps if there was any stridence or insistence from either or both of their parents that Lilly and Jack must have wandered into the woods, and then leading them to a spot where they said they found a piece of Lilly's blanket (and a boot print?) leading that way, and they were acting extremely nervous (even the LEO on the dispatch sounded really nervous when relaying that part about the piece of blanket they were shown by the parents, it was chilling to me)...
Just saying there could have been indicators LE picked up on that maybe they hadn't wandered into the woods, so they did their searches with the tracking/scent dog and found absolutely nothing to indicate that's where they were, so they weren't going to pursue "wandered into the woods" any further.
I believe if none of the searchers ever found a thing & the tracking/scent dog never signaled to picking up either of their scents anywhere beyond the yard where they played, that gave LE the information they needed to direct their resources towards pursuing other possibilities.