I would pick the fanny pack in four different colors, one for each person. Each person then becomes responsible for loading up their own pack, with a little assistance from the (adult) others if necessary.
I've gotten fanny packs as cheap as $1 at a dollar store, but they were flimsy and not good for water rides.
You can get fanny packs at a variety of price points and sizes. For younger guests, a relatively simple fanny pack for snacks and whatnot would be appropriate--I won't go on about what might go into them (that's for another post). But the adults in the party might want to invest in a bigger verson; they're often carried at outdoor/outfitting shops, etc. The larger ones have enough space for two water bottle, a small guidebook, sunblock, etc., to balance out the size of the smaller fanny packs.
I myself don't use a fanny pack, but a day trip or field bag. Its strap is padded for my shoulder, I can strap it across my body for security on the rides or at night, access to my things is still convenient (plus, it loads a guide book from the top) and its also appropriate for a wider range of sightseeing activities (in black it looks like a casual purse). Mini backpack/purses fashionable with the younger set might work as well. Just some alternatives
Always Stalking the Mouse!