Everyone likes carrying things different ways. I'll give you my thoughts on what worked for us----
Our first trip to WDW our twins were 2 1/2 y/o and both in diapers. I carried a backpack bag with everything we needed in it. By day 4, my shoulder was killing me from having to constantly transfer the bag from my shoulder every time we got on and off a ride, on and off the bus, sat down to eat, etc.
The 2nd trip to WDW our twins were 3 1/2 y/o, and both still in diapers. This time I brought the freebie diaper bag I got from the hospital (because if stolen, this would be no big loss). I packed enough diapers, wipes, juice, snacks for the day in it and left it on the stroller while we were in the parks. I also threw in 2 shirts and 2 pants that were inexpensive and would be no huge loss if stolen. These were just extra clothes in case of an accident. I used a small travelon bag across my shoulders to carry my money, park tickets, bibs, antibacterial wipes, stickers to occupy the kids when we had to wait in line or wait for food, and camera. This worked MUCH better for us. My shoulder never hurt, and I didn't have to carry all that stuff with us on every ride. At the end of each day, I just re-packed supplies for the next day.