McKays, Discovery Cove is incredible - one of the best days of the holiday for us. But it's owned and run by SeaWorld so you may not want to go if you have ethical issues with SeaWorld.
Mozzchops. kids are very often underwhelmed when the big announcement is made - it happened to us and to lots of other people I know! I'm amazed that you hated Universal! What was it that you didn't like about it? I don't do big coasters but USF and IoA are still my favourite parks after MK.
Peaz, you mentioned getting a 2 or 3 day ticket - you can get these from Undercover Tourist but you may well find that the 14 day tickets are cheaper. Certainly a 14 day Universal ticket is cheaper than buying 2 single gate-price tickets.
Pigley, re. what Disney parks are covered, that depends on the individual ticket - if you're buying a multi-day ticket from the UK then it will include Magic Kingdom, Epcot, Animal Kingdom and Hollywood Studios. The 2 water parks are nearly always included too but I have seen ones where they aren't, so just check before you buy - the vast majority will cover every park. As for whether a week is enough, it depends if you want to see everything - if you do then you'd have to go at it fairly hard to get everything done in a week. To see everything you really need 2-3 days at Magic Kingdom, 2 at Epcot, 1 at Hollywood Studios, 1 at Animal Kingdom and 1 at each of the water parks - so that's 7-8 days and it would be very tiring. The parks are BIG and there is a looooot of walking (10 miles a day or more!) I usually end up having to take paracetamol because my legs hurt from all the walking :)