About aamir k***
A good way to think about it is to consider the characteristics you'd like your child to have. These include physical as well as psychological traits. Things like their personality, sensitivity, sense of humor, talents, intelligence, and other intangibles.
Personal letter
Start off with a sweet salutation like, "Dear darling," "To my love" or invoking their special nickname so they know this letter is all about them. Don't be afraid to get mushy to really set the mood. Next, tell them why you're writing a letter instead of just signing a generic card.