Tell them what you expect. Let the kids know that completing their homework is mandatory, not optional, and that it's something that they'll have to deal with as long as they are in school. Emphasize how most adults had to do their homework as kids, and that the majority of people who grow up to be successful in life were disciplined students as children, and did their homework throughout their school days.
Offer them help. Let kids know that an older person is willing to assist them as long as the kids know that no one else will finish their homework for them. Sitting with a child and monitoring her work, and being there to answer any questions she might have, is a good way to let her know that doing homework is part of getting a good education. It can also be a family-bonding experience, as an adult gets to share in the experience when a child learns new things.
Speak with them about when is the best time to do homework, and then establish a regular routine. Getting input from the kids on the time to do homework will make them partners in the process. After agreeing to a regular block of time, let the kids know that they have to stick to it. With a solid structure in place, it is easier for kids to fall into a regular homework-completion routine.
Convey to the kids how doing their homework now will help them throughout their lives. Make sure they know that homework is something that all students have to deal with at all educational levels. If the kids already have a career goal in mind, emphasize to them that they will have to have good study habits and complete their homework in order to finish school and achieve their goals.
Establish consequences for failing to complete homework assignments, and then follow through, if necessary. Have the kids understand that if they fail to complete their schoolwork, they'll lose a privilege that they enjoy, such as playing outside or watching TV. Not being able to take part in something they like can be a strong motivator for kids to finish their homework.