I've wondered about that, too. And in light of the mother and grandmother already going through one missing person case and the arrests in the family, I would completely understand that. They had to know the scrutiny and experience would be ----.
So why report her missing at all?
If they KNOW she's dead, why put themselves and everyone else through this? Why not pack up and move to Utah -- relocating for his job. They'd already been talking about it. In's on record. They could move anywhere and sell the property in time. Wherever they decide to move, they would be a new family with 3 boys, no little girl. No one would know unless the boys eventually tell. And I suspect they would and their lives would be horrible from now on. But if escaping prison is the goal, why report Summer missing?
Unless they truly hope to find her and save her life, why do this?