I have to disagree with some responses to doing the mindless drudgery of math computations that are below a childs' level. I'm a computer programmer/analyst, and sometimes my work is exciting and challenging, and sometimes it's mind numbing drudgery. I don't know anyone who doesn't have some drudgery mixed in an otherwise exciting, challenging job. I think you are doing your kids a disservice by not teaching them coping skills to get through the mind-numbing times. One way to get through it is to time it, and make it a race against his/her own best time. Then share some cookies when it's done.
That said, you are still right to advocate for more challenging work. One reason I've heard for a lack of acceleration is "but what will the child do in 7th grade if they complete 8th grade work in 5th grade?" The answer I got from GS's gifted teacher is, "But we spiral around and get more in depth each year". She never got it that GS was already going deeper on his own!
