Well, I don't have my packing list handy, but some of our must haves that come to mind for the littles are:
for our 6 month old: pacifier clipped to onesie, bottles, crib ordered in room, rental swing delivered to hotel, extra onesies, baby food, bib, disposable spoon in baby bag, hat, sunscreen, chewie toy, stroller he could recline in, stroller fan (unfortunately, it broke on the plane, but it would have been nice to have in the park), small receiving blanket to put on stroller when he is facing into the sun.
for our 2 and 4 year old: an extra stroller for the 2 year old (it's nice if it gives you room for the soft sided cooler in a basket), with those stroller links to link to the baby stroller when only one parent is pushing (they are great!), a personal fan, baby wipes to wipe their faces and necks when hot at the park, suncreen (lots!), hats (with straps if you have active ones), snacks like granola bars and water in bottles, camera -- those faces they make are precious, autograph books with sharpie marker (I bought little notebooks at the dollar store before we left)
I'm sure I forgot a lot of things, but maybe this will help.