Okay, here are my suggestions

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Rent an offsite condo - go to the Orlando Other Attractions board to find out the easiest and cheapest way to rent at Bonnet Creek, Vistana, or Windsor Hills (those are the 3 most popular offsite resorts areas that can be found cheaply). You should be able to find a week in a 2 bedroom at one of the 3 for $1K or less (probably less).
I'd break the trip up as follows...(assuming your trip is one week)...
I'd get 1 preferred annual pass and 3 seasonal passes for Universal. You would be able to plan a free return trip back under these tickets, you'd be able to park free, and you'd be able to take advantage of their Sept BOGO 1/2 off daily CS deal (they've run the past few years). I'd do 3-4 days at Universal parks (if you do water parks, I'd definitely fit in the extra day)
I'd plan on one-two special Magical Dining Month evenings - this event is ALWAYS in September. There are some very nice places included, some which handle kids (like Emeril's, Morimoto's, etc).
I'd get 1 day Disney passes and 1 day party passes for the Halloween party at Disney's Magic Kingdom. Not sure what parks are highest on your list, but I'd skip EPCOT and HS myself and do 1 day to see Animal Kingdom and the party to see Magic Kingdom (where you can be there 4pm-midnight).
Total ticket costs as planned will be about $1.9-2K (this is gonna be your highest cost), BUT you'd have a free trip for Universal already paid for next year AND parking handled at Universal and you'll get trick-or-treating and special events at MK!
So, if you went Sunday-Sunday, I'd do a sample schedule as follows (subject to change based on your check in day and eventual scheduling of Disney parties)...
Sunday - Arrive, Check In, Groceries and Resort Time (maybe Magical Dining Month dinner)
Monday - Universal Studios
Tuesday - Magic Kingdom Halloween Party (resort time in morning)
Wednesday - Universal Studios New Water Park
Thursday - Universal Islands of Adventure
Friday - Animal Kingdom
Saturday - Repeat of Favorite Universal Sites (maybe 2nd nice Magical Dining Month dinner)
Sunday - Drive Home