one water bottle, - OK, but get a string and get a kid to wear it.
sunscreen, decant into a very small bottle, or get those sunscreen wipes? To be honest, we don't carry it, we put it on in the morning and forget, but we don't travel in August
autograph books, Kids carry there own, in their own bum bag. This helps too if you split up , we had too many traumas where Kid and Dad spotted a character, while Mum was off getting drinks with the autograph book in her bag - much safer for DD to have the autograph book with her at all times.
digital camera and video camera, We don't bring the video, but I have a bulky digital camera - I founda nice large bum bag (was only about $5 in
Walmart) that I can carry it in. you can almost definitely find one that will take both your cameras. They do a nice (but pricey) one in the camera shops at WDW.
ponchos, We get those tiny ones from Walmart that are $1, they are basically disposable, but each person can easily fit one in their bumbag
snacks I've stopped carrying much - we just tend to prefer to buy food and the snacks just get carried around. I do have emergency snacks for the baby in her baby bag
, sunglasses, on strings around necks
wet wipes, sanitizer Other than wipes for bottoms, I don't carry either - I am not a sanitizer sort of person. You can get tiny packs of wipes though, and I can see how they could be handy - but I don't believe in sanitizer, a few germs never hurt anyone.
spare batteries and memory cards and video tapes These all go in the camera bum bag
I don't think you are taking too much, you just need to spread it round your family rather than lugging a huge backpack.