DON'T pack your own stroller if you can help it. I've heard so many stories about how thankful folks who rent a stroller from one of the reputable places near WDW, on how easy it is and how they were happy they didn't have to worry about it. Like I said though, if you can help it.
Well...you're a mom, so you're probably thinking of everything already, but the big thing I'd say is don't over pack. essentials we don't leave without: Anti-Chafe of some sort, moleskin, bandaids, neosporin, alcohol/medicated wipes, handi wipes, 16oz drink mix-ins (Aldi's has 40 hawaiian punch ones for 4 bucks), our own double-walled 16oz waterbottles (cause I'm nor paying for water), sharpies for character autographs, autograph book of some sort (we used the Disney Junior Encyclopedia of Animated Characters), ziplock bags (sandwich, squart, gallon and a couple 2-gallon) dollar ponchos, trash bags to use as laundry hampers, battery chargers for phones, power strip for the room...
...I know I'm forgetting some stuff, but you get the idea
Those are the things we definitely have for sure.