I don't get the feeling that the grandparents intentionally destroyed evidence. And there is no indication that they did so after the realized Caylee was gone. The cans were moved prior to that. The pants were pulled and washed prior to LE being called. We all heard the 911 calls span a couple of hours. Hindsight is really easy but I think they have a flake of a daughter, they are use to her outlandish behavior, they get a letter telling them their car is impounded, they get it back, it reeks.... I don't know that most natural reaction of a parent is to think "my daughter may have actually killed someone, maybe even my granddaughter, don't touch this possible crime scene". Probably more of a what the hell did she do, where the hell is she, and why am I always cleaning up her mess.
They have let police in the house, volunteered the laptop, let cadaver dogs in the yard. They could have lawyered up right away and made this really difficult for LE. They haven't. Stupid and regretful actions, yes, intentionally hiding evidence I don't know.