How can I thank parents for doing a fantastic job?

I'm a teacher. In my role, I come across all types of parents. However, there are just some parents that are doing a darn good job of raising their kids (in my opinion and relative to other parents I encounter). For instance, I have one student whose parent encourages him to be polite, gives him chores, and administers consequences when he does not meet expectations. As a result, this child is extremely polite (asking if he can carry things for me, etc) and responsible. I have another student who lost his father - his grandmother is his caregiver now. She seems overwhelmed with the responsibility, but constantly puts his well-being first (seeking counseling for him, etc.) I feel a little silly asking this, but I would love to recognize these parents in a private way. In a quick email, a letter, etc. Something along the lines of "Keep it up - your kid is awesome!" However, as I do not have kids of my own, I'm not sure this would be a welcome gesture. Any suggestions on whether this is a good idea and/or how to phrase it would be welcome.