We found out before our most recent trip and my parents were meeting us at WDW.
Our original plan was to have them wait in line to see Mickey and then run back to my parents (who would be sitting elsewhere) and say, "Mickey just told us we're going to be big brothers!" My oldest son didn't want to do that (he's getting too mature for stuff like that, apparently lol!) so we had them just flat out tell them.
Another option - which we did the last day for a picture instead of the day my parents got there - was to get a blue and pink balloon for you and your hubby and see how long it takes them to figure it out. We did this for a picture - my kids gave the camera a thumbs up and DH was holding the blue balloon and kissing me on the cheek, while I held the pink balloon and had my hand on my stomach (the photographer came up with the pose). It turned out to be a really cute picture.
And for my in-laws, I got a Disney Christmas ornament and personalized it "Love, Grandbaby #5" and that was fun to see them see that.