I asked the Disney Moms about this and got this reply:
For the 5 p.m. performance you need to dine between 1.45 p.m. and 2.45 p.m.
For the 6.45 p.m. performance you need to dine between 3.30 p.m. and 4.30 p.m.
For the 8.30 p.m. performance you need to dine between 5 p.m. and 6 p.m.
HTH