Are you wanting to do the same gift for all ages? If so, then maybe sunglasses, themed snack like a rice crispy treat, lanyard, shirt, character socks, a hat, some kind of tote bag/backpack/drawstring bag to carry stuff around the parks or a wet bag for swimming suits, flip flops for the pool, a sweatshirt/hoodie for any cooler nights, a themed throw blanket, slippers, themed Disney ear hats.
If you're okay with giving different to the older ones vs the 8yo, then for the child, Disney tsum tsums, small Lego polybag builds or Disney lego minifig, a plush, goggles, autograph book if they are still into that, a Disney mad libs or joke book, Disney Guide to the Magic for kids book (gave that to my 7yo and she loved it), glow sticks/glow ice cubes/any kind of glow thing, Disney shaped pop its, waterproof stickers for the water bottle.
For adults, a water bottle holder (one that clips to a belt loop or on a strap that you carry kind), portable stick battery charger thing, waterproof phone pouch, valet tray (fabric tray that can snap in the corners to hold loose change, keys etc- helps to keep your stuff organized at the end of the night), lightsaber chopsticks (on
amazon), Disney keychain, themed luggage handle wraps for the last day (makes it easier to spot luggage in baggage claim).
Have fun! When we went, we would have one of the adults stay sneakily stay behind in the room for a minute after everyone walked to the bus stop and they would leave out a small gift for the kids to find when we got back from the parks. We made little tags saying they were from Mickey and left little stuff like tsum tsums, tattoos, pins, a small plush, and little figures. They loved it and I'm sure your family will love your surprises as well.