Hurray! Have fun at Disney World.
I have had luck with both my own parents and my in laws quoting the WHO guidelines. I haven't had to deal with direct disapproval (DD is only 7 weeks), but little less than helpful comments like: When are you going to teach her to take a bottle? She can't be hungry again. She needs water. You'll eventually have to give it up and use bottles because she'll be too hungry to breast feed all the time. How long are you going to breastfeed?
I've also taken to starting conversations with: "I learned this about breastfeeding today..."
I also keep trying to remind them that billions of humans have been raised on breast milk successfully...and unless your daughter can sit down and eat a steak dinner, she still needs milk!
Final card -- I spoke to the pediatrician before vacation, and he said it was SO important that I breastfeed because LO will be around new germs on the plane/in the park/etc... The doctor INSISTED that I breastfeed.... :-)
If you do use the nursing areas, which sound really nice, take your MIL. Maybe she'll see other women nursing older children.