I think the first thing that you need to decide is the length of your trip - IMHO you can do WDW reasonably well in 7 days - if you want to do anything else (Harry Potter, the Beach...) then plan on at least a 10 day trip.
It is hard for those who have visited Disneyland several times to really comprehend when they begin to plan for their first trip to WDW, but there are some really important differences between Disneyland and WDW.
When planning a trip to Disneyland: TIME, MONEY and ROOM SPACE are pretty easy to figure out - after all, the 2 parks there are contained within a couple of hundred acres and surrounded by plenty of room options. However, when planning a trip to WDW, the weight that you give TIME, MONEY and ROOM SPACE are more difficult to figure out.
Disneyland: You can do Disneyland one day and California Adventure the 2nd day and then go back to your 'favorite' park for the 3rd day. Yes, in 3 days - you can cover pretty much all of Disneyland/CA. You don't even really need to purchase any sort of Park Hopper (although some do, of course). Also it is not necessary to stay on-site - in fact, one of the best cost-saving strategies is to stay in a very nice nearby offsite hotel.
WDW: These strategies don't translate well to WDW. For example - it would be nearly impossible for a newbie family of 6 to do 5 days at WDW - say, 1 day for each for Epcot, MK, HS and AK and back to your "favorite" park for the 5th day. First off, all the parks are far apart, bigger, and the often humid weather can slow down everyone. So, IMHO - given the ages of your children - a Park Hopper is a necessity for your entire family. Also, take a look at the MYW ticket prices - along about Day 4 or 5 - the ticket prices slow dramatically in price per day - and multiplied by 6, it can be a big cost saving to slow down and take the time to really enjoy WDW, as opposed to rushing across town to say, see Harry Potter.
With the ages of your children (no babies or toddlers) it would be fine to stay offsite (but nearby) in some sort of condo where there was a washer/dryer (less to pack) and a kitchen. If you go that route, though - you have to factor in renting a car at the airport and making a grocery stop for breakfast items. You will need to contrast that with staying on-site by perhaps renting DVC points (but only through a reputable firm who has been in business for years) at OKW or SS (2 bedroom with washer/dryer & kitchen) and getting some groceries delivered by a grocery delivery service. However, with this option, you will not need to rent a car as you can all use Magical Express to get to/from the airport.
If you do want to rent a mini-van for a side trip to Harry Potter, Alamo & National have WDW on-site rentals and you can drop it at the airport.