This is a sample. Yours is written for your child from the details you give us.
Dear Leo,
Your very first tooth! I did three loop-the-loops over your roof when I heard the news.
I have looked at a great many teeth, and yours is a beauty: bright, clean, and brave. I can tell you have been brushing every night. Thank you. Shiny teeth make the best bricks for my moonlight castle.
I also heard you read a whole book by yourself this month. That made my wings sparkle. Readers are my favorite kind of people.
I have tucked your tooth safely in my pocket. Sleep well, and take good care of the new one growing in.