Mine is still in diapers so I don't know if all of this is applicable. I have my "purse" which is a small sling style backpack basically packed for the parks, plus for the plane I had some EarPlanes (didn't really work for me) gum and suckers. For the parks I have a small blister kit, a small flashlight for DD which I have only used for finding small toys under resturant tables, a very compact raincoat for DD that fits in a snack baggie, baggies of disposable bibs and disposable changing pad covers, baby wipes, antibacterial wipes, diapers, a compact change of clothes for DD, camera, phone, itinerary, copy of park tickets, emergency contact numbers. I keep a seperate pocket for my medicine, tylenol for adults and kids, antibacterial lotion, sunscreen, and chapstick - I can empty all that into my quart ziploc at the airport.
Then I have my carry on backpack just for the plane which is toys, books, snacks, tippy cup, jackets and more diapers and wipes. If luggage gets lost we are stuck with the clothes on our backs! I do cross pack - everyones stuff is spread out between the checked baggage just in case. I also have a stroller and a carseat, so I can't handle clothes in the carryon too. We hired a towncar this trip so I didn't have to worry about my luggage being delayed from Magical Express.