Hopefully they do have fun, I know I greatly enjoyed camping at their ages.  My kids are all younger than that, but they enjoy it too!  I don't think it is short sighted and it sounds like your have the basics taken care of though. If you are camping with electricity, the occasional use of tablet or DVD player may not be too bad.  Especially in the larger parks like Yellowstone where it can be a couple hours to drive between campground and attractions.  Some would say electronics are a no go, but I find they are a fun distraction at night, bad weather days, and in the car.  Toys, coloring, and board games can work too if they like them.

Kids generally love the outdoors, but may be a little weary of some of the odder things about camping if they are new to it.  Bugs, pit toilets, camp showers, the sounds at night, wildlife, etc. can be a foreign experience for first timers.  My kids (and me at their age and older) can spend/have spent hours collecting leaves, throwing rocks in a creek, hiking trails, finding sticks for the campfire, playing in the water (beaches and streams, not geysers or other thermal features which you should never approach off the boardwalks aside from Boiling River which is the only one you are aloud to go in), and watching the wildlife.

Soap is regular dish soap, think Dawn, not dish detergent like for your dishwasher.  Have another tub to rinse off and then a clean towel for drying.

Some daypacks have whistles built into the top clips otherwise you can buy a whistle from REI and many other online retailers.

Yellowstone specifically has some amazing things to see, but make sure you keep your food inside the car or bear box as not to attract bears.  Be prepared for slow foot and car traffic in the summer and early fall.

Superusers do not speak on behalf of REI and may have received
one or more gifts or other benefits from the co-op.