I would be disappointed but not surprised to learn the parents are guilty.
I think the scenario is quite simple - sedate the children at bedtime so they go to the restaurant safe in the knowledge the children are fast asleep. For some reason the child reacted badly to the sedative - perhaps an overdose - and it was enough to kill her. Or she vomited and was so heavily asleep she didn't wake up and drowned on her own vomit.
I think the child is either at the bottom of the sea, weighted down, or under a new road or building.
As I said, I would be disappointed if not shocked if this is what happened.