Hi Kristin,
My heart warmed just reading your story. I also co-parent with my kids' dad, and even though we are no longer together, we have continued to share family experiences, including vacations, for the benefit of our kids. I commend you, your parents and your ex for trying to create such a wonderful surprise for your children during your upcoming trip.
Now let's put on our thinking caps (and Mickey ears) and try to come up with a magical way for your kids to "discover" their Dad in the World!
Do your kids have a favorite attraction or experience they enjoyed in the past with their Dad? My kids definitely inherited their sense of adventure from their Dad, and during our past trips to Walt Disney World they have ridden
Expedition Everest together up to five or six times in a row. (Mom normally bails after the second run!) If your kids have a similar affinity for a certain attraction, I think it would be fantastic for their Dad to be standing at the entrance of this attraction when the rest of you arrive. Be sure to have the camera at the ready, because I am sure you will want to record the reactions!
Another option is to meet at a special favorite restaurant. You could plan to arrive with the rest of the family and discretely inform the Cast Member at the podium that your kids' Dad is standing by to make a surprise appearance. Then, once you are seated, he could walk up to the table to join the rest of you for a celebratory meal.
Another cute idea would be a poolside meet up. If your Walt Disney World Resort hotel has a water slide, you could dispatch your kids to the slide, arranging for their Dad to be in the pool to greet them one by one when they splash into the water. Now that's a Splashtastic surprise!
Whatever you decide, I have no doubt this will be a trip your kids will remember for a lifetime!
Trish