overall cost

All Disney all the time, no rental car, with meal plan, flights to Orlando: 
$4182.56 = $10 tips for shuttle + $1098.30 park tickets + $1015 hotel + $1261.26 meal plan + $113 tips at meals + 685 flights

Two Disney resorts, one water park, two Universal Parks, mid-sized rental car, parking, lunch and dinner at restaurants with no meal plan, 2 bedroom condo, flights to Tampa: 
$3822.90 = $260 rental car + $102 in parking + $1703.90 park tickets + $599 condo + $350 restaurant lunch and dinner + $100 groceries for breakfasts and snacks + 644 flights + $64 BWI parking

Adding an extra person: 

$81 change to condo with extra bedroom (http://www.vrbo.com/696468)

$170 Extra flight

$398 Park tickets for extra person

$337 restaurant lunches and dinners, and breakfasts from grocery store for extra person:

$986 Total cost to add an additional person, assuming the person shares the rental car and no additional parking fees at the parks are required.

Research:

Meal plans must be purchased at the same time as the resort package is booked, and must be booked through Disney, ie, can't use any bargain websites for booking the resort, so no discounts.

My first 2 attempts were made assuming Dave's kids were 9 or under, but in fact they are both 10 or over.  This will make the meal plan cost more.  But, since Rob says Dave won't pay tax on the gift of the meal plan, will the cost even out?

1st attempt:

$1108 park tickets (6 day, 1 adult, 2 kids, park hopper)
$1212 lodging (7 night, Art in Animation)
$892 meal plan (7 day, 1 adult, 2 kids, Standard)
$685 flights (Southwest, saturday to saturday)

$3897 Total.

2nd attempt from Disney Go:
https://disneyworld.disney.go.com/cart/
$2693.43 (park ticets 5 day, 1 adult, 2 kids, park hopper)(7 night, Art in Animation) (Disney dining, next step up from quick-sevice plan, 7 day, 1 adult, 2 kids, Standard)
$685 flights (Southwest, saturday to saturday, booked 6 weeks in advance)
3378.43 Total

3rd attempt:
Rental Car: $263 intermediate, Toyota Corolla or similar, with automatic and a/c, total with tax, intermediate from Alamo. parking $17/day, gas $20 for 200 mi at 30mpg and $3/gal.

Total cost of car: $263 + 20+ (17*6) = $385 or Disney shuttle = $28 in tips
Tickets: Total $1303.98---OR-5-day Disney 3 adult with water parks is $1098.30 
Resort: Disney All Star Movie hotel is $1015 -OR-  $931.51 for Caribe Cove 2 brm condo w/kitchen
Food: Disney Dining Plan $ $60.04 per adult per day = $1261.26 + $113 in tips, -OR- $911 Total restaurant food + $100 groceries for breakfasts and snacks = $1061 which includes tips at all table-service restaurants.  
All Disney all the time, no rental car, with meal plan, flights on S/W: 
$4200.56 = $28 + $1098.30 + $1015 + $1261.26 + $113 + 685 

Two Disney resorts, one water park, two Universal Parks, mid-sized rental car, no meal plan, 2 bedroom condo: 
$4074 = $385 + $1303+$690 + $911 + $100

5 days at Disney parks including water parks, mid-sized rental car, no meal plan, 2 bdrm condo not on Disney property: 
$3869.3 = $385 + $1098.30 + $932 + $911 + $100