I can't exactly remember, but the bear-proof storage boxes they look 60" long, 36"deep, and 36" front to back. Some campsites have "goalposts" with pulleys...bring your own(?) plus rope...to hang food at night...in double-freezer Ziplocs to minimize scent. There should be regs posted on "Bear Aware" camping. Yup, bring that spotting scope, or borrow one & binocs from good friends and neighbors. If you can, get an iPhone adapter to take pics through the scope. A good tripod is worthwhile, as it gets windy especially past Mount Washburn on those winding-road pullouts...and especially in the afternoon, not so much at daylight, if you can get the girls up early enough.
As for fishing, go online to www.nps.gov/yell/planyourvisit/fishing.htmpublications.htm
The "paper pamphlet" regs have one-page on "Where to Take Young Anglers" w/ good suggestions. Remember, bend those barbs down with needle-nose pliers, and my kids started with the quintessential Zeb-33....Mepps Spinners, Hell-divers, small crank baits, diving Rapala minnows, Yozuris, or Tsunami soft baits. You can also tie a tandem set of nymphs below an old-fashioned red/white bobber. Up by Mammoth, after taking in the elk on the manicured lawns, get early to Joffee Lake, as this is the only lake in the Park...where 11 year-old and younger can FISH w WORMS! and trout love live worms. The NPS/YNP pamphlet that you can pick up at any of the entrances suggests, for kids: Mammoth Area-Blacktail Deer Creek( where we've flyfished often ), Indian Creek near campground, Gardner Rvr(again good flyfishing and eager cut ties) at the picnic area between the North Entrance/Roosevelt Gate and Mammoth, and of course, Joffee Lake. Lake area-Yellowstone Lake along Gull Point Drive or at Sand Point. Grant Village area-Aster Creek near (gorgeous) Lewis Falls, Lewis Lake shoreline. East Entrance-Middle Creek, Sylvan Lake. NE area-Pebble Creek, Trout Lake. Old Faithful area-Goose Lake, Firehole River (early mid-morning or late afternoon-YNP sometimes closes the Firehole midday due to stress/rising temps) near picnic areas, and Nez Perce Creek in the deeper holes-this is a cool little creek that flows under the road dumping into the Firehole, either fish upstream or downstream in deeper holes-watch out for the small bubbling geysers right on the banks-they are hot/warm! Madison area-Gibbon River near Tuff Cliffs.
BTW-I almost forgot about a better series of maps. "Backpacker" magazine loves these maps-Look up "Earthwalk Press" Hiking Map & Guides-they have one for YNP and GTNP. The YNP map shows what burned in the 1988 fires, advice/area for "Bear Country Hiking", Backcountry regs, Topography, trail mileage, suggested hikes, etc. These maps are waterproof and virtually indestructible.
Back to fishing regs-as long as the girls are directly under your supervision, children 15 and under may fish without a permit, as long as the adult has a valid YNP fishing permit. OR, they may obtain a FREE permit, that must be signed by a responsible adult (you); and they can fish "without direct adult supervision". My twenty-somethings still have their first permits, when they were 8 and 10, and your girls may want these as memories to hold on to for the future!
Enjoy!