Cherry, we can only speculate. but they were trying to keep something(s) secret. We don't know what evidence/information has been withheld that may be contained in medical records, and one can only guess what those poor children were going through in that house... Their mother was very ill for a long time, and their father had a pretty demanding career I expect...
The grand jury felt that the parents were responsible, yes, you are correct.. but what exactly were they responsible for? Well, obviously felony child abuse of JBR:
PR and JR "did unlawfully, knowingly, recklessly and feloniously permit a child to be unreasonably placed in a situation which posed a threat of injury to the child's life or health, which resulted in the death of JonBenet Ramsey."
And this:
PR and JR "did render assistance to a person" with the intent to prevent their arrest or prosecution, knowing they had "committed and was suspected of the crime of murder in the first degree and child abuse resulting in death." Who were they rendering assistance to, Cherry? Someone else or each other?