You didn't mention (or I didn't see it mentioned) if you and the other family HAVE to stay at the same resort. If they really want to stay off site, and your family really wants to stay on site, then your family can stay in a regular room at WL since your youngest is still under 3. Its a wonderful resort, and is decently priced if you can get a standard room.
Just a small suggestion.....We've traveled with other families many, many times. At first we'd all try to stay at the same resort, request rooms close to each other, all that. Now we tend to each pick what our family wants and we stay at different resorts quite often. After spending some of our park time with them, and having many meals together, its nice to be able to each go to our own resort for some down time. As much as we love our travel buddies, we also enjoy our own family time while on vacation. We don't feel the need to be with our extended family round the clock on vacation.