I think if I am right they have lockers at the loos inside the entrance. Keep your dry clothes there (2 sets as you just know you will do 2 trips on the water rides!) then you won't have to haul them around the park.
Ditto for snacks, prices are extremely high. That way you can snack 'free' during the day and treat the kids with a bought lunch. I think the Pizza Hut deal was good as it was 'eat all you can' so if the kids were not keen on one slice of pizza they could easily try another. Also had salad bar and bottomless drinks.
The 'jump the queue' tickets are good value and means you can get the 9 year old through a few of the 'bigger' rides faster without boring the 19 month old to tears in the queues.
Stick a label on the 9 year old with your mobile number on... just in case!!
Agree to pay for one photo of the day for the 9 year old on their fave ride, that way you don't get nagged half to death after every single photo op. You can arrange to pick them up at the end to save hauling them around and getting them wet. We do fridge magnets instead of photos and the kids love them!