Here's the packing list for each of my children (in addition to things I pack for us all to share, like foodstuffs and First Aid supplies, or things I keep with me until we arrive, like the kids' Magicbands) -- they each have their own 21" rolling suitcase (we travel carryon only), and the things they need in-flight are either packed into a small backpack of theirs, or tossed into my or DH's "personal item."
Actually, the items listed below don't completely fill their suitcases, so the extra space is occupied by breakfast stuff on the way down (e.g., mini cereal boxes, Pop Tarts) and souvenirs on the way back.
Lay out ahead:
1 - Travel outfit (including bulkiest clothing, e.g., jeans, hoodie, sneakers, tee, sunglasses)
2 - In-flight entertaiment (books, electronics with chargers, ear buds, cards) and/or snacks (e.g., granola bars, apples, empty water bottle with filter), and "comfort items" (my 10-year-old son may be all macho and independent during the day, but he still can't sleep without his favorite blankie at night, so it travels with us)
In ~21" rolling carryon, in packing cubes:
3 - several warm-weather outfits (1 per day plus 1 extra, or half that amount, if doing laundry mid-week), with each set of shorts, tee, socks and underwear stacked, folded and carefully rolled together into an "outfit burrito" so the kids can simply grab one from a drawer and get dressed by themselves each day
4 - waterproof hiking sandals (for park touring with wet rides, rainy days, visiting the pool, or just for a change of footwear)
5 - pajamas
6 - swimsuit, swimshirt, goggles
7 - sun visor or baseball cap
8 - one "nice" outfit, if visiting a signature restaurant
9 - autograph book(s) and pen
10 - child-sized Dollar Tree ponchos (or adult-size ponchos, trimmed down) for Kali River Rapids and/or rainy days
11 - glow sticks and/or mini-flashlight, if desired
12 - small dopp kit or pouch with hairbrush/comb and non-liquid toiletry or hygiene items (e.g., deodorant, toothbrush, floss, chapstick)
In carryon, in outside pockets:
13 - 3-1-1 bag with sunscreen and any liquid toiletries (e.g., toothpaste)
14 - shorts to change into mid-trip, if traveling from cold weather