Expect the best of your children and they wil expect the best of themselves. However, don't set overly high expectations or demand overly high efforts. Assess what your child can do and encourage them to strive for this level. This may be a level that they achieve sometimes and you want to help them maintain that level more often. Or, it may be a new level that is a real step up for them. Either way, ensure it is realistic and adds value to their lives.
What sort of world do you want your children to grow up in? It is your children that will shape the world of the future. It is you that will shape your children. Decide carefully and shape accordingly.