Basing on multiple people's answers, there are multiple answers already have been provided which I can't retort on but I would like to give my own opinions from my point of view.
Considering the 'Hero' concept is only formed due to being blessed by an almighty being, the generic trait of the hero is mostly derived from the blessing he had recieve and not liable to be transfered to future generations. So, taken from that, any future generations born from the hero are unlikely able to inherit any blessed abilities the hero has yet can inherit his traits and skills to future generation, albeit with alterations due to being a new generation from the hero himself.
The demon king is of a different case as most demons are likely form from blood inheritance and like how noble society works, they are mostly derived by bloodlines and as such, each future generation would likely inherit their key bloodline skills and traits.
The cross-breeding would likely happen as humans are genetically inheritable to be able to breed with humanoid races as explained by their fast growth rates. Factoring that, it is unlikely for humans to be unable to not breed with demons unless it is medical issues and whatnot.
However, as most people had said, they are able to create a powerful child, yet their traits and skills are not fully taken from both sides, especially on the heroes side as genetically inheritence doesn't transfer their blessing from they were heroes. As such, most of its traits would likely come from their demon's side and mostly take form into more demon-like appearance however they still inherit the human's fast growth speed which would compensate their lack of power boosting abilities and even developed new traits that come from both parents side as being a halfling.
I would like to end my talk by recommending to buy my book at my patreon page which encapsulates my discoveries and research on fantasy beast to zombie researches. The link is currently unavailable as I am still working on some fixes regarding deleting old works posted there.