Model a love of learning. Have fun and make learning an enjoyable part of life. Remove too many unnecessary distractions.i.e. Put limits on tv and video games etc. Try to avoid using reward too much. It tells children that learning is a chore that requires bribes. The hidden message for kids is it must not be worth doing if I need some kind of payoff.
Reinforce good habits. Be realistic about their work. Encourage them to seek feedback from teachers and help them learn that constructive criticism is a great way to grow as a learner. Encourage their involvement in academic clubs at school or outside school (i.e. reading clubs in libraries). Get them involved in some type of sports or athletics. Exercise helps build healthy connections in the brain allowing them to think more effectively and efficiently.
Diet is also important, so removing foods full of artificial dyes and processed junk can help. Healthy body, healthy brain. Alpha omega 3 fatty acids are also supposed to be linked with brain growth, so maybe add a supplement to their diet.
Focus on the process of learning and your kids should be fine. Grades in and of themselves have no inherent value. In fact they are often the reason some kids get discouraged at school because they are a way of comparing kids to each other. Some kids get turned off by the comparison. Other children would rather not try and do poorly than work hard and still possibly not do well, so they don't try. It's a useful way for them to protect their egos. If they don't have good grades they just say I don't like school or I don't try at school. It's better than not feeling smart.
However, having realistic but high standards for quality school work will always be appreciated by teachers and other evaluators and will help your kids throughout life. At the same time, focusing on their effort and the process can help. Some kids become good learners later in their school life. If their is encouragement to love learning rather than pressure to succeed they are more likely to do well. If it becomes a power struggle, then it is more about them asserting their autonomy than disliking school.