I believe lots of people have their children take a nap during the afternoon, even older ones, so that they are awake enough to see Dreams and spend an actual evening at the parks at least once during their stay.
I don't have children yet but I personally would do it that way too. Maybe even take a little nap myself, it's not like I would have something better to do waiting for them to be awake so might as wel take a powernap and be fresh and shiny for Dreams.
So bedtime moves up a little but I do think it's important to keep the amount of hours of sleep kind off the same as on normal days, just split up between a quick nap and their nighttime bedrest. If they sleep too little you're stuck there with very whiny, naggy, overtired kids and that way neither you nor them would enjoy it. And Disney can be quite exhausting, even for us adults, so I can't begin to imagine what an impact those busy days must have for the little ones.
Europeans tend to eat early tough (well the Northern part, in the South of France/Spain they eat later than here in Belgium for example) so I believe the 6.30 time slot is always busier. I myself would find 9.30 too late for dinner/tea, I like having a few hours of time after eating to walk around and let my meal digest properly before going to sleep. I find that walking around always helps me lose that "Oh God I ate way to much" feeling (and trust me you'll have that after Buffalo Bill's with all the food and drinks they shove in your face

) so try to avoid having to go to sleep quickly after dinner.
We went to Buffalo Bill in october 2012 in mid season to the 6.30 showing and it was fairly quiet there. The CM's put everyone in the "good seats" and directed everyone to a place with good view so that no one was sitting in front of those iron bars that shield of the arena from the seating area at eye height. It's quite a big arena tough and I believe you have a fairly good view from every seat, even if it would be crowded.
If you really like your peace and don't mind eating late, I would go for the 9.30 slot.
