Arrive early - get there a half hour before the gates open.
Download the app before going so that you can see queue times and check which rides your kids are tall enough for. Your 4 year old may or may not be tall enough for the bigger riders (mine can only do vampire out of the more exciting rides).
Bring a packed lunch and snacks for the queues.
Smaller rides often have smaller queues (but not all the time - the trucker type rides are long queues all the time).
There are shows and meet and greets if you want them - gruffalo and Cocomelon meet and greets. Times on the app.
Zufari is quite a long walk and a long queue usually.
My kids love the tigers.
You get wet on the river rafts so don't wrap anything around your waist or wear a longline cardigan as water collects in the raft and when it goes up the track all the water run back to your where your bottom is. We usually do this when it's sunny or towards the end of the day. Often a longer wait than the sign says for this one.
Sea storm is surprisingly nippy.
Take a buggy if you have one - your 4 y/o may be whingy if you do a full day. Plus you can use it to lug bags/coats around. Don't leave valuables in it though. Lockers are available for a charge.
Toilets can be grim - bring hand sanitiser. Taps in toilets are sometimes very hot.
Preempt the queues and warn your kids so that it's better than they're expecting. I bring bubbles for the queue - your 6 yo may be beyond this but it may help
There's a Lidl literally up the road or an M & S petrol station just down the road. A massive Tesco in Leatherhead just off M25 - just in case you want to buy food en route.
You will have a fab time. Enjoy it! I hope the sun shines for you! It's great for these ages