I think a villa makes the most sense for your family. If the majority of your vacations were spent in the Orlando area it would be by far the most economic and simple answer to your situation.
In answer to some of your questions, most Americans don't get as much vacation time as we in Europe do. For the most part a more regular use would be a couple of 5-7 day breaks a year.
As to the number of points you require (should you go for
DVC), you have to ask would EVERY holiday be to Disney, it's a great destination, but I do think you risk burn out with such heavy use. Even if I was to choose Florida as my destination for all those trips I wouldn't stay on DVC for all the time. The front of the line access at Universal makes staying at one of their excellent hotels a very desirable option for at least some of the time. If I were to vacation in the manner you are expecting I would limit myself to one 5 day stay each trip (as a maximum). I would also take into account that while at the moment you would need a 2 bedroom unit as the girls grow up that is not going to be the case. When they have left home you would require only, at most, a one bedroom much of the time. With weekends being much more expensive than weekdays points wise you can limit your DVC time to only weekdays, booking into USF or a beach break at weekends. On that basis you could say at the moment you would need about 150 points each 5 day stay (OKW would be less, BCV more) . Three or four trips at 150 points mean you'd need a maximum of 600 points. That comes in at an initial outlay of about $57,000 if you bought the new 50 year duration DVC or if you bought a resale it would run about $45,000. The annual cost would be about $2,500 a year. With banking/borrowing and not staying on site EVERY visit you could quite easily pare that down to 400 points so an outlay of $38,000 from Disney or $30,000 for a resale and annual cost AT THE MOMENT of about $1600. Still a pretty sizable chunk of change.
Coming from the UK I'm certain you'd mostly be coming Easter, Xmas, August and one of the halfterms (October or Feb). Xmas and Easter are higher cost items at DVC but I think 400-500 points would provide you with a workable option to much of your requirements. It would allow you take a break from Florida 50% of the time and you could then stay much longer on DVC without having to rent out your points or let the go to waste. It also would allow for when your kids no longer want to go on holiday with you and your wife. It would mean you could get a smaller unit for longer, again not forcing you to rent or waste points.
Looking at your overall situation I don't think DVC is the best answer if you plan to spend so much time in the Orlando area, but it could provide coverage for a large part of your holidays and still give you flexibility to visit other places. Having owned both ( a villa and DVC) I would say that having a villa (anywhere in the world) does force you to visit the same place year after year. Whereas with DVC's banking and borrowing you can take a break if you start feeling jaded. While I had both, I still made good use of my DVC, sometimes I would stay 2 weeks in the villa and spend the last 5 days on Disney at DVC. Other times I would go visit Vero beach for 4-5 days to have some beach time. At other times I took a whole vacation at Hilton Head's DVC property which made a really nice change from the themeparks every year.
If the dues were $4-ppt then your yearly dues would be 8000-
