Universal isn't known for its kid-focused atmosphere. They are however famous for throwing an award-winning Halloween event each year. The Universal target demographic is teenagers and up.
I fear of what would happen at a Universal kid's party. Either it goes horribly wrong or it ends up being creepy in a bad way/sad. A small kids themed Halloween event like Not So Scary confined to mostly Seuss would be like the birthday party that your parents threw you with that creepy clown.
This is not an area Universal needs to explore. As you said, every other Central Florida park has it covered. Let Universal stick to doing what they do best; clearly they have the crowds they want from it.
You make some very good points. And customer service would be slammed with guests claiming they "accidentally" went into HHN with their kids, stayed long enough to see all the houses and Bill & Ted, and now DEMAND their money back because little Johnny peed himself in fright.
That said, let the Grinch "take over" Seuss Landing (maybe with Thing 1 and 2, they creeped me out as a kid), have the Super Villains* out in force in Marvel Island, some special kind of non-horror, non-alcohol** theming in WWoHP, throw in some candy stations, I think the event
could work. And I do think Potter gives them an edge over similar events--the only thing that could separate them from the pack, really.
As for someone else's point about Marvel, I think they could definitely use it as "generic NY street" for a HHN event--and the house in that area already felt separate from the Island. Just no more dead Spider-man. And my guess is they could use it "as is" for a more kid-friendly event, with the super villains they already have rights to.
* Did they keep the costumes from Maximum Carnage? Carnage himself might be off-limits for a kids event, but maybe Shriek? Can't remember who else was in there.
** Maybe 3 or 4 years ago the shot girls disappeared overnight, didn't return until the following year. Rumor alleged it was because a VIP had been offended by scantily-clad women pushing alcohol--and that said VIP was JK Rowling. Never been confirmed, but I wonder if it was that aspect of HHN the WB execs were referring to, not an opposition to Halloween in general.