I am going to WDW with a 15 month too! We are going in december! I am so anxious about it!!! It's going to be her first trip, and I just can't wait to see her reaction! I have no idea about these issues of baby food... Better get informed about it.
ok, this doesn't pertain to food or soy milk, but when we went w/our 18mth old we found a few things very handy to have. these items were sunblock, handiwipes, a spraybottle/fan (you can get them at discount stores).
I don't think they do carry soy milk, I remember looking for lacatse free milk and they did not have that. I recomend you stop at the store before getting to your resort. While they carry many things at the resorts, they are really over pirced. If you don't have a car,a town car service like Quick Sliver will take you to from the airport to the resort and they give a free store stop. That way you can stop at a store and get the milk and other things you might need. Having snack food is very helpful for young children that age. (you can also by your dipars and wipes there and save some travel space.) Quicksliver also has free car seats for children.