The kid was right... All along (Screenshot from Vanilla)

Before Frozen Throne was released, there was a secret in the second undead mission of reign of chaos. You have to use demolishers to clear a path way through the trees to a hidden grotto. Once you reach the grotto, a picture of two pandaren flashes on the screen. I think there's a hydralisk on the same map.

Also Illadin's blades in WC3 have pandaren crests on them.