We had 4 kids on each of our previous trips and now we have 5.
So we've got a pretty long list of stuff that we carry.
I've got an over the shoulder type camera bag that holds my DSLR, 3 lenses, and a miniDV camcorder.
Then we can usually fit everything else in 1 backpack:
Diapers, wipes, change of clothes for the youngest, sunscreen, sunglasses, wallets, ibuprofen, chapstick, 4x6 photo book filled with index cards with all of my trip planning info (itinerary, dining reservations, park hours, etc., etc.), ponchos, flip flops if we're at AK and plan to do Kali, and as many water bottles as we can fit in the bag.
Weight isn't too much of a concern because we've always had at least one stroller with a basket.
For our next trip, I think I'm going to try to look for a backpack that can work as a camera bag and still have room for all our other stuff. Having one bag to deal with would be a lot easier.