At this time keep your reservation & continuously check for a better time.
FWIW, only you know your kids and their eating/sleeping patterns. MY DD6 would be beyond hungry at 9:05pm, maybe not sleepy in Disney. She has a different sleep pattern in WDW.
Also, the time depends on what your day is like. We arrived in WDW in June around 1pm after leaving home @ 5:30am. So our eating/sleeping pattern was already off for this day-which we knew. We utilized DME, hotel check in, appetizers @ Sanaa, Disney transportation to park, etc. We had an BoG ADR @ 7pm (only one available---and worked for our schedule being our travel day), but at 9pm she was sleepy and ready to go. Wouldn't even go on 7DMT (we had FP+). 7pm is later than what we usually eat, but it worked for our travel day.
IMO, BoG is a great experience. We have eaten there 3 times & DD6 loves it! However, if you are going to have tired, hungry, grumpy kids (due to the time) it's not worth it. This could potentially dictate your next day too. However, if your kids can handle the time with being reasonably happy/content eating at this hour & traveling back to resort. Remember meal will not be over until ~10:30pm. Walking to front of park, Disney transportation wait/or walking to car---this could be from 11pm-Midnight depending on crowds, mode of transportation, transportation wait & travel times.
This is your vacation. You will not see everything this trip or the next. But you will have your memories from this trip. Always remember to have fun and enjoy the Disney experience!!!
