One stop faster (f/2.8 vs f/4) is not going to make a big difference. Even a f/1.4 lens might be marginal. I usually use my 35mm f/1.4 in low light and crank up the ISO to 3200 or 6400.
At some point, a external flash is going to be needed.
So your saying that to return my 70-200 f4is for the 2.8 version will not give me the performance that I am looking for and a flash will most likely have to be used. Last year I had the 17-55 2.8 with iso 3200 on a 60d still was only 1/30 and really too slow. So I figured 1/60 @ iso 6400 would be better but that was with what the equivalent of the 24-70 ii is. I am just trying to figure out if I will need fill flash to keep a more tolerable iso say 2000 then a flash will be needed whether f1.4,2.8,4.0. Did I get that right. I also don't like really shallow DOF since the goal is to have the kids, present, etc in focus but then will my 430 ex ii be good or the 600 since it has more power and is made for my focus sensor? Sorry for all the questions but I really value everyone's opinion's. I think I will post a pic from last year so I can give a visual example for more accurate advise.