Try to lay off of the rewards and punishments thing... Sometimes you just need to "trick" them into learning.
If they have homework, do it with them. They do, of course, need to get this done. So if this is the problem, just make it routine! Sometimes they need a break between school and this, though, so don't force it right after school. Just say to them, "We will need to spend x minutes on homework tonight. You are x years old and old enough to decide when you would like to do that. Your bedtime is x o'clock, therefore you need to have your homework started by x." It is teaching them to be responsible and accountable. They will really feel excited about the grades they earn this way! This is kind of how my parents were with me.
This summer I needed to review multiplication and division with a boy going into fifth grade. We played multiplication card games, multiplication dice, multiplication dominoes, math stuff in the car, multiplying and dividing license plates. He would tell me hwen he was ready to do it, I just needed to give him the reminder in the morning. This was a child who despises school and school work, and once given responsibility, he was very good.
No matter what, do NOT underestimate your children! They are so much smarter than you think! They'll come around. The first couple weeks are usually pretty hard. Stay strong!
Ashley
P.S. PM me if you need any game ideas!