Once you have ordered the letters for your child (or grandchild), start a thread with his/her name as a title (you don’t have to use the full name) and include some details about him/her to inform Santa of what is happening in his/her life. Santa might try to include some details in his next letters. There is no guarantee as it might also depend on the theme of the letter.
You can update the thread as often as you want. Santa will keep an eye on it.