will you be taking a break mid-day for your kids to rest? if not, keep in mind that a disney vacation is stimulating, exciting - it tires them out a little more quickly than usual when they are little. until this year, we always had to take nap breaks with my kids or they would "lose it" by 8pm. they always sack out immediately after putting them in bed, though.
the show lasts around 25 minutes, I believe...so you'll be exiting the theater around 9pm. it depends on where you're staying as to how long it takes to get back to your hotel...also, how quickly you can walk to your bus stop (or car, if you're driving). We stayed at AS Movies when we went, and I think in all, it probably took us an hour to get back to the hotel after leaving the theater this last time (via bus).
if you go...try to sit close to the area that will be let out first...toward your right as you enter the stadium. we've gone twice to Fantasmic...first time we sat more toward the left side of the stadium, and it took FOREVER to get out...plus people were pushing in tight on each other to get toward the other side of the stadium to get out. we had to hold the kiddos really closely, and even then I was afraid to lose grip on them in the crowd. reminded me of bourbon street on mardi gras day-without the alcohol.

the 2nd time we went we sat in that reserved area, and it was much easier to get out...still crowded, but since we were some of the 1st people out there wasn't as much shoving.... maybe you could find a spot near that reserved area.