I can't think of a plausible scenario where they would cover up an accident - unless maybe hard drugs were involved and they would face a long time in prison if LE found out about the drugs.
An accidental drowning, a hot car death, a car accident - I can imagine one person panicking and deciding to hide the body. But two of them, both thinking it was a good idea? Or four of them, if you think they all knew about it? No way. Didn't happen. Whatever happened, they were scared of serious charges and a long time in prison. Scared enough to refuse to call an ambulance, scared enough to go driving around with a dead body, to leave their child without a decent burial, to concoct a story, to face constant scrutiny from LE and from the public, to lie to all their friends and family about what happened and leave them without closure... No way was it a simple accident.
All just :moo: