While you may not directly be able to motivate your children, you can help them set long-term goals that will give them the direction and motivation to succeed in school. For example, if your children are taking computer classes, have them visit your workplace or a friend's office to see exactly how important it is to be able to use a computer. This could motivate them to do well in their computer classes. Career guidance from a school counselor could help your children to explore various career options and put them on the right track.
Exploring and setting long-term career goals should be helpful in turning things around for your children. You also need to talk to their teachers to see if you can find out more about why they're unmotivated. Do they have the skills needed to handle their classes? Are they finding school fun? Do they have friends? Do they participate in school activities? Do they have ongoing conflicts with teachers or fellow students? Lack of motivation can stem from problems in any of these areas.
Be sure to visit colleges with your children and encourage them to participate in sports. Either of these activities could motivate them to make better grades.
It's possible, your children are not challenged sufficiently in their classes and find the whole school experience a waste of time. Or they may be enrolled in classes that are too difficult for them. Don't discount the role their attitudes play in affecting their school performance.