Children who are just learning to write, for example, will misspell words and form their letters imperfectly. Focus on helping your children understand the idea of thanking others and on the effort they put into doing a thank-you note. Do not worry about having a long or perfect-looking final product. Writing thank-you notes is a habit, and habits are not learned overnight. Expect to encourage and support your child's writing thank-you notes for a long time before they are ready to do so independently.
Children learn to be courteous and responsible by watching their parents.
A purchased thank-you note can be used if it also includes a personal message of thanks. Include a date and send the thank-you note as soon as possible after receiving the gift. I recently attended a wedding shower and within 48 hours received a thank-you note in the mail. Her note was very nicely written to express her enthusiasm for my gift and her pleasure that I had attended her special event. I also remember receiving a thank-you note three months after a wedding.