I stayed with a host family several years ago who are still my "Japanese family" that are very dear to me.
Maybe you already know how much English they can or cannot speak. However, if you are unsure, or even if you are, I would keep the letter very simple. Just a few words saying thank you for letting my daughter stay in your home. I repeat that just a few kind words are all that is needed. I think the most important thing to include is a few family pictures. Have your kid bring plenty of pictures of your whole family. I mean grandmothers and pets, the host family will love it. It is a good chance for your kid to bond with the host family as well. Breakign the ice can be tough, and the pictures will help.
Also, you may have been told already, but gift giving is common in these circumstances. An "omiyage" given by your kid to the host family is good manners. It should be something actually made in your region. Not like a t-shirt made in China that says something about your state, I mean a real object made in your region or city whatever. Small treats are a usual choice like regional chocolates or desserts. Don't spend a lot of money, make it edible and it will be a big success.