They will have a line open for character breakfasts. You get in line, they may or may not check you confirmation number, but they usually have your name on a list at the least, and they will send you back to the restaurant. Be sure to get there a little earlier than your ADR. Sometimes with rope drop ceremonies you can't get into the park at certain times (atleast I've heard this about Animal Kingdom - you can't get in after 845am and are let in at rope drop.) I haven't done a pre-rope drop ADR at H&V (we had 950am), but have done them at all the other parks so I don't know about DHS's particulars. I would try to check in no later than 830am for your ADR time.
If you are staying at a Disney Property, you can catch character breakfast buses that run earlier than the regular buses. For DHS (Epcot & AK), they begin running around 7am and run about every 30 minutes or so. Or with your ADR time, you should be able to catch the first regularly schedule bus for DHS that should run at 8am.
Disney sounds like a great place to enjoy Halloween! Have fun!!