Doing your own researches beforehand will pay you handsomely!!.
At Animal Kingdon would suggest you ride the Kilimanjaro Safaris asap after you get there. The Kali river rapids ride is good but you may get soaked. The Dinosaur ride there is lame.
Not sure if the Expedition Everest ride will be open but if it is when you get there ride it!!.
If you use the Fastpass system (available on most rides) you can only get one lot of Fastpass tickets at a time and every member of your party will need a ticket to ride (you can ride between a certain hour timeframe e.g 10.15-11.15). It does save quite a lot of time.
The parks at the weekend do get very busy - would do the parks therefore on weekdays. Think the Magic Kingdom's busiest day is Monday.
"The Unofficial Guide to Walt Disney World" by Bob Sehlinger is worth reading before you go and as a reference when you are there.
Sea World is worth a day out.
Dolphin Swim at Discovery Cove is good but you need to book this in advance before leaving the UK. It can also cost you a fair bit. You will also need to provide them with picture ID (i.e your passports or a photocard driving licence) for all members of your party. Their arrival procedure is somewhat longwinded (you are photographed) and was not very well organised when we went. Any jewellery including earrings must be removed (lockers are provided).
Downtown Disney early evening is worth a look-see.
If you are one for shopping I would recommend the Mall at Millenia, the Florida Mall and the Vineland Premium Outlets.
Both Universal parks are well worth a visit; think your 11 year old may particularly like both parks.
Your 7 year old should like it as well.
Be prepared for a long walk at Orlando airport on arrival. Do ensure that all of the relevant paperwork required by US immigration is completed well ahead of time (you will be given these forms at check in. Do not do what some people do and complete the forms whilst standing in line!!). You will each need a green visa waiver form completed (you as the parent will need to complete and sign one for each of your children) along with the customs form (only one of these is required per family). All adults in your party will be fingerprinted and photographed on arrival into the USA.
I take it as read that all the children have their own individual and machine readable passports. They need these for entry into the US.