I suspect that the kids were "cooperating" in the beginning. Some in fact, may have been talking to police before even talking to their parents. I think it's likely that in the beginning, some had no clue this was going to go on for so long. Anyone who is innocent probably thought she was going to just show up at any given moment It probably wasn't long however, before things began to get serious, beyond "surreal", parents retained attorney, and kids were told not to talk.
As I've said before, much of what these kids say can incriminate them. If they provided drugs to her (or perhaps were even with her when she took something), that may be grounds for leading to her death. Even if LE give them immunity, her parents could likely still use it against them in a Civil Suit, which can still ruin their reputation. These kids may have much on the line, even if they are innocent of any wrongdoing beyond drugs.