Drowning may be a better term! Lol! Mix a Type A with a WDW vaca and it's obsession city from here on out. Part of the fun for me is the planning anyway so I'm really loving that there is so much to learn. Our oldest grandson has been asking to go back to WDW since the day after our last trip (3 years ago when he was 4) and we are finally going to make his wish come true. I am so excited! There is nothing like seeing the world thru the eyes of a child, in my opinion. We are thinking of waiting until the day we are planning to leave to actually tell the children. I wonder if anyone else on these boards has ever done that and how it turned out.
To answer your initial question, here is what I would do:
1) Try to hone in on a few specific weeks.
2) Read the
point chart (there is a link at the top of the page that says
DVC Point Charts).
3) Figure out what resort you might want to try for, and how you want to break up the accommodations. While Grand Villas are nice, you would do a little best cost wise if you go with a 2 bedroom and a studio (and this might give you a little more privacy).
4) Add up the points for the rooms you would like, using the point charts, and then calculate the cost by figuring that you're going to pay from $10-$14 a point, depending on which resort, rooms, nights, etc...
This would at least give you some parameters as far as cost. The points per night vary tremendously. A 2 bedroom at Bay Lake Towers in November can range from 38-60 points per night depending on what night of the week and what view.
You will probably have the best luck at Old Key West (biggest rooms and reasonable points, and generally lots available outside of 7 months), Saratoga Springs or Animal Kingdom Lodge.
If the points on the higher end of the cost range are within your budget, I would consider using a rental service. They can walk you through most effectively, and the peace of mind might be worth it. If you want to save a few bucks, you can "work" the Rent/Trade board here and look for somebody who will work with you.
As for surprising the kids at the last minute, there are entire threads of discussion around these boards, some have turned out just like you would see in a commercial, and some have turned really sad. There are some funny
youtube videos if you want to do some searches about parents surprising kids with trips to Disney. I think it sounds like your 7 year old would be thrilled, so a surprise would probably be a great idea--if you can keep it a secret, of course.