Authenticity in magical letters depends on sensory dissonance. When a child sees a note written on the same A4 paper used for school homework and in a handwriting style they recognize from a grocery list, the brain immediately flags the discrepancy. To maintain the magic, you must decouple the letter from your daily domestic identity.
Trusted Compass suggests shifting the focus from the content of the message to the physical delivery. A truly authentic Tooth Fairy letter should look like it traveled from a different realm, utilizing materials and scales that suggest a creature only a few inches tall created it.