I do not think the Daybell children were involved in any way. At all. Now, maybe they started to suspect something odd, but I think they were unwilling to dip their toes in that possibility.
I think Chad buried the kids with the help of Alex at the very least and maybe both Lori and Alex. I think they did it when Tammy was gone. I also believe the police finally got the warrant based on all the little things that came together: Bonfires, Modular home permits, concrete slabs being requested, technology data from cell phones- what if Tylee's phone shows pinging at Chad's house or neighborhood AFTER she was presumed missing? I just think they were able to get a convincing list together and the weight of all the components on that list were enough to allow the search. The cadaver dogs went straight to the graves. They started to dig. BOOM.
This is my own opinion.