Just as one may find freshmen and graduate students taking the same college course, so, too, with Sonlight Curriculum programs. Your choice of "year" should be based more on content than on the number we happen to have attached to the program. You may wonder how this can be.
The primary reason: Great literature (unlike most textbooks!) is appropriate for a wide range of ages. I have a master's degree. I have been fascinated and educated by materials in the Sonlight® Kindergarten program.
You could use unique programs for each child, but you also have the choice of using one program for two or more children. Which ultimately means that you save the time you would have spent juggling the extra program(s), save the money you would have spent on it (them), and avoid much of the frustration that comes from being pulled in multiple directions.
Another one of the great benefits you will reap if you combine your children into one History / Bible / Literature program is that all family members learn together. Wonderful conversations will occur at the dinner table about a book you have shared together. Everyone, down to the preschooler who sits on Mom's lap as she reads, will be able to participate, and everyone will learn, not just from the books, but from each other, as a result!
However, if the age difference between your students is too great, we do not recommend that you combine your students within the same program. Your family can still study the same basic events of history together if you choose History / Bible / Literature programs that cover the same basic time periods.
Please see the attached PDFs for visual samples of how you can combine curriculum resources to teach two or more children with a single program.