As a teacher, in addition to what everyone else has said (be specific, avoid empty superlatives, mention how she helped your child learn and grow, keep it less than one full page), I'd love to have evidence of little things I did that meant a lot to the student. We have so many interactions with so many kids, it's impossible to remember every detail, and it's good to be reminded that the everyday words and gestures matter. Oh, and if she did a good job staying in touch with you as parents, that's good to mention too.
Also, thank you for having this impulse. A letter like this is more thoughtful and would mean more to me than any gift card or teacher appreciation tchotchke.
Oh, and I'm not sure if you'd do this, but avoid any mention of "for a first year teacher," which could sound condescending (even if it wasn't meant that way).