As I recall LE didn't know where the children were for some time. Everyone hoped the children would be found alive somewhere - anywhere.
It wasn't until LE were digging through Chad's text messages that they came across the 'raccoon' text, which was sent right around when Tylee was last seen, that tipped them off. In it Chad texted Tammy about shooting and burying a 'raccoon' in their 'pet cemetery', and also burning 'limb debris' in their fire pit. That sounded strange compared to his other short texts to Tammy, and gave law enforcement an idea of maybe what happened and where to look. Still, they didn't know where this 'pet cemetery' was, until they talked to (If I remember correctly) Tammy's sister, who was able to describe the location and pinpoint it on a map of the property.
They also didn't know where JJ might be, until they got on scene and were able to see signs of a potential recent burial near the tree/pond. This is the "short grass" Hermosilla testified of the other day. They started digging there too and found JJ.
