You may want to let them try and work out how long it will take you to travel somewhere when you get in the car, by telling them what speed you'll be traveling and how far away your destination is.
If you find that your child is still really struggling after a number of efforts at home, there is one final thing you can do. The option of hiring a personal tutor is becoming more popular throughout the States. Personal tutors can help your child get to grips with things they never used to understand and provide a more personal form of education. The personal tutor will become more of a friend to your child, than a teacher, meaning the teaching style will be vastly different to that of their school. It's one of the best ways of helping a struggling child.