Originally Answered: Disney vs Universal studios Florida. How long in each?
Your family obviously has previous experience in theme parks. What y'all probably DON'T have is experience in doing new theme parks day after day after day. I know it's tempting to try and "do all the parks and rides" since you may not be back. But really, it'll turn you into mush. All these parks are extremely stimulating, after 2-3 days everybody will be gorked out, particularly young kids. Also, these parks are not like Knott's Berry Farm or Six Flags. There's a lot in between the rides to keep your head swiveling around. Some of the rides themselves are quite intense beyond the spinning and dropping.
I do think you're cutting your time at Disney short, the Magic Kingdom and especially Epcot can be two-day parks. The MK has lots of rides (and lines) and the sheer size of Epcot means doing it in one day is pretty insane. (It was originally planned as two parks, that should give you a hint.)
My advice would be to buy a three day ticket to Disney, and go to the Magic Kingdom and Epcot first, they're more unique. Get to Animal Kingdom and Hollywood Studios if you are done with all the other parks, including Universal's. I love the AK but a large part is based on slowing down and looking around, and if you can't do that, you miss a lot of the park. Hollywood Studios has great thrill rides, but Universal has many more. Of course this only works if you're not hurrying through other parks so you can "finish" all of them. There's not a prize or medal for doing all the rides and parks. Trust me. You're already missing Sea World and Busch Gardens, so why try to cram stuff in for the sake of cramming?
Get a one-day park-to park ticket for Universal. (You can add another day if you don't see all you want. The same goes for Disney.) Islands of Adventure and Universal Studios are right next to each other so you may be able to do them in one day. Matter of fact, you might want to go there before Disney. Universal is more like Six Flags, with an emphasis on big thrill rides and not much "magic" outside them. Be prepared to split your party up. Many of the rides have minimum heights, and most are pretty intimidating. Even if the 9 year old can handle some of the extreme coasters he/she may not be 4'6. Check the ride requirements before you get in line if you think the younger kids will freak if they get psyched up to ride and then booted at the station.
In short, get tickets, go to the parks, and THEN decide how many days you'll spend where. There's no need to schedule your vacation. You won't see everything you want to see, period. Go and enjoy, stop when you've had enough. If more people did this, they would get far more out of their vacation.