Most people think Casey was guilty of at least some negligence in her daughter's death? Most people feel that an average mother could have/would have averted this death?
What are the possible ways that death could have happened?
1) Drowning.
2) Deliberate asphyxiation
3) Murder by parent
4) Murder by Casey
5) Automobile accident
6) Accident
7) Death by animal
Of these, plausibly only a deliberate murder (2,3 or 4) would explain the cover-up.
You can add in 1,5, 6 and maybe even 7. There has been enough not saying a lot but enough cases where the parent has been arrested tried and convicted of manslaughter in accidental deaths where covering up an accident might seem like a good idea at the time especially if she told her father an ex cop and he remembers a few of them.