Why not do both? You could enclose two notes or cards in one envelope, with a personal thank you from you and you child.

Ask your son to brainstorm things that he he really liked about his teacher. Was there anything in particular that he or she was especially helpful with? Did he enjoy reading or doing art projects with his teacher? Suggest that he thanks his teacher for his or her hard work, and any specific field trips, projects, or lessons that stood out to him. As long as the letter comes from the heart, it will be well-received and appreciated by his teacher. Try having him write it out on scratch paper first, so that he can get help with sentence structure and spelling.

Since his letter will most likely be on the shorter side, you can also write your own letter, giving a more detailed explanation of the positive impact that you think his teacher has had on him, and anything in particular that you appreciated about his teacher. Did he or she go above and beyond setting up a holiday party? Was his teacher great at sparking your son's imagination? Along with a general, heartfelt "thank you," give some specific examples of things his teacher did that you were grateful for.

Here are some previous ideas users came up for celebrating Teacher Appreciation Week; you may find some good ideas here! http://www.education.com/question/child-celebrating-teacher-appreciation-week/

Also, I'm including a link for a teacher thank you note activity that uses an empty cereal box. Not only is it cute, but it's eco-friendly, too!