This is a general list I found on an WDW packing list I found online. I don't know who to give credit to. If you'd like the entire list, I can email it to you. We just edited it to our own needs, you can too.
just a couple of extra items not on the list:
1. strap for sunglasses, not really a back pack item, but we found them great for kids going in and out of buildings and rides. Don't have to collect the glasses or worry they will get left behind.
2. small ziplock bags for food items not finished but you want to take with you, like unfinished suckers, cookies, muffins etc. My kids like to half-eat something and then go looking for the rest of it later on.
I have a 9 year old boy and he wore his swim trunks to the parks...That saved us from carrying extra set of clothes. They dried fast. I think it also depends on the weather.