I would be up front with them, but I disagree with PPs who've suggested placating them by offering one meal together. I don't know about you, but my DH and I don't get that many romantic getaways just the two of us without any kids. When we do, I'm really not interested in wasting even one of the few intimate meals we have during that time catching up with my ILs (and I mostly like my ILs). Also, from what the OP wrote, it sounds like these ILs would want them to "show them around" Disney, which would be more than just one meal.
Anyway, I would tell them you're going to be there, because as one PP said, you don't want this to turn into a sitcom plot ducking around corners and constantly worrying you're going to run into the ILs. That will ruin the romance of your trip even if you never encounter them--it'll be like they're with you even when they're not. Just say, "What a coincidence! We planned a Disney trip at that time too, a romantic getaway for just the two of us." If they suggest getting together during your stay, simply say something like, "Any other time, that might be lovely, but we've been so looking forward to a few days of 'just us' time. We'll see you when you get back." If they're going to be "disappointed and hurt," that's on them.