you can simply say things like in your letters:
"you are a pleasure to know"
"You are a joy in this amazing class"
"I know that the class will be amazing with you"
"You are exceptional and I like that you are my little one."
"having you in this class makes life easy for me."
Make your letters very simple...if that are little ones --aged 5-7 ...write one or two lines ...if they are 7-9...make the letter about 5 lines.. it is enough...
Write something special about a child...a thing that ensures you that he is going to love your class and love studying......
Hope this helps you
Regards
Khadooy