Should the parents be considered sinful, or was their son's death after the hernia operation a matter of divine decree and destiny?

All praise is due to Allah, and prayers and peace be upon the Messenger of Allah, and upon his family and companions. To proceed:

You should be patient and seek your reward from Allah. We ask Allah the Almighty to reward you for your affliction and to replace it with something better. There is no sin upon you for what transpired, because taking the means and seeking treatment is Islamically مطلوب (required/encouraged), and lifespans are in the hands of Allah, the Glorified and Exalted.

And Allah knows best.