William:
I usually recommend that folks get to your restaurant reservation about 15 minutes ahead of the reservation time. You aren't guaranteed to be seated exactly at your dining time but my experience has been, especially at lunch, to be pretty close to it. However, arriving near that window or even a few minutes afterward shouldn't result in your table being given away.
My concern for you, though, is that not only is
Nemo: The Musical about 40 minutes long, but it can take 10 to 15 minutes to get out of the auditorium. Additionally, it will take you 10 to 15 minutes at least to walk from the auditorium in Dinoland over to Tusker House which is located on the opposite side of the
Animal Kingdom in Africa. If I were in your shoes, I would opt to either change your dining reservation or look for another show time for Nemo.
Wishing y'all a magical vacation!
Whitney