Next, you can make a schedule for them. If you organisation out work time and play time, then they hit a goal; something to work towards. If all they know is that they hit to sit down and do work, for some presented amount of time then they probably will not want to do it. So provide them something to look nervy to, a light at the end of the tunnel, a reward for their focus and hard work!
Finally, after they hit complete their work, not only can you reward them with free time, but you can award them with academic medals & trophies. There are many to opt from at Crown Awards to match the specific subject where your child was having trouble. This way they can look back and remember how they overcame their frustrations in that one subject area.