When are you going? Think about what your kids will enjoy most. We're just back and spent more time in waterparks than I expected (it was unseasonably warm!). The kids enjoyed them more than trudging around the dry parks in the heat. They'll be included in your tickets if you have the UK 14 day ones.
Get a subscription to touringplans.com, think carefully about what attractions you'll want to visit, and have a plan for each day. At your kids' ages, I'd say they'll enjoy Magic Kingdom and Animal Kingdom the most. You might want to invest in Genie+ for a day in Magic Kingdom for some of the rides; the queue for Peter Pan, in particular, can be very long for such a small-child-friendly ride.
Check out the food options in advance and have a plan for that, too. Use mobile ordering wherever it's available, it's a godsend. Try to eat outside of peak mealtimes if you want a chance at getting a table in a quick service restaurant.
Bring water bottles to the parks and refill them at drinking fountains, which you'll find outside most restrooms. We did that for three weeks solid and saved a fortune, we were really thirsty all the time and park drinks are crazy money.
Snacks will probably help in queues, and if you're staying after dark, glowsticks to play with.
Your 5 year old may very well nap; between jet lag, heat, excitement and exercise, there were days when my 12 and 11 year olds would have napped if I'd had a buggy!