Just wanted to thank you for your responses. I have tried explaining but maybe not in the exact words from above. I will try OHGrandma's approach which is sort of what I had come to but maybe not for a whole week. We do leave off the TV at times so I knew that was an option from the start. Apparently OHGrandma's option 2 only took about 4 days, LOL! All of the grandparents were visiting this weekend for DD4 (just) bday so we did nothing. Monday night I had no arguing even though it is spring break (no vacation for us, but yay on the shuttle launch!)
My GT denial usually consists of times when DD8 just gets things on Aleks that seem really easy to me so of course she should get them easily, right? Then my friend who used to teach 5th grade tells me that her 5th graders would still have issues on the concepts that are easy for DD8.