Children love to get gifts from Grandma and Uncle Henry, but few volunteer to sit down and dash off a thank-you note. Indeed, many adults drag their feet on this task. But these simple notes are so cherished by the gift-giver, and they teach a child that generosity must be acknowledged.
Be sure to have your child write it himself. Most can manage something by second grade, some much earlier. Don't fuss too much if misspellings creep in as long as the message will be understood. Just a few lines will do the job. If you have a special-needs child who cannot write, she might tell you what to say and she can decorate the card. (I have found packages of blank notes with matching envelopes at Hobby Lobby that are perfect for this.)
Some card companies are now offering preprinted thank-you notes for children. These brightly colored cards say things like "Dear ____, Thank you for the _____. Love, _____" While I applaud the good intention of the companies, this is a bad idea. A child should learn to write a note from scratch. Show them also how to complete the envelope. It's surprising otherwise how old a child can get without knowing how to address an envelope, where the stamp goes, or why she should put her return address on it.