A task that can be done while standing up and moving around is generally better than one that requires your child to stand still. Try to determine your child's best times during the day. Then try to schedule the most difficult tasks or the ones needing the most concentration during the better times. Be consistent in what behaviors you expect from your child. Give the child praise when he or she completes something that took great concentration.
Kids are most likely to repeat the behaviors that get the most positive feedback.
If your child has homework and is on medicine for ADHD, have the child start the homework within 1 hour of taking the afternoon dose. That way your child will get the maximum benefit of concentration. Don't wait until right before bedtime, when the child is irritable because of sleepiness and there is no concentration-improving medication in his or her system. Be sure your child is getting enough sleep. Sleep is so important for a child.