Not contagious. Researchers have identified certain genes that are associated with a higher risk of some cancers but not all. They have also been tracking rare "cancer"families that have higher than expected numbers of different cancers in their family tree. The best way to get your kids risks is to discuss this with your doc and possibly a clinical geneticist.
Which cancer? Some certainly are but most currently do not have a hereditary component. Please ask you doctor about the specific type.
Need more info.. I would like to help but I need a little more information. What type of cancer are you referring to? Who in your family has had that cancer and at what age?