I think it's a great idea! What Universal could do to avoid the confusion issue is just be upfront about it. Have signs that LITERALLY say "For Kids and Family" and then another that says "Adults Only" and then have a brief description under each. Have TMs posted at both Entrances explaining/reminding guests of the age limits and stuff.
OR
Have the Kiddie stuff from 5-7 and keep the HHN at 7:00...or even 7:30. By this you'd have all kids that shouldn't be at HHN gone and the adults who do plan on going to HHN can just walk over next door.
Have Half IOA opened. Starting from Seuss, going through Potter, and ending at Toon Lagoon. There could be a small parade in Seuss, ghosts in Potter and Death Eaters walking about for Photo Ops, and A show in Toon Lagoon theater. There could even be a mini Minion Halloween Party in Port of Entry.