First of all; I really like your render! Lighting is great, very nice deer as well. One thing I would do, though, is lower the original grass object a bit (5-10cm), so the snow looks thicker as only grass tips protrude after populating. Or have them sit on a power fractal shader that has a little displacement and offset (say 10cm displacement and half of that as minus offset), attached to final shader, and 'hanging loose'.
To get frost on objects, you have to add a 'white' color (or a mix of bluish and white, with some translucency and/or luminosity, like can be done in a default shader) to the parts. If you just add a surface shader between the default shader and the part, it will be totally white, so you could mask that by something like a tiny fake stone in a certain coverage, so you get all little points of white, or a tiny power fractal, or both. All through a transform shader set to world/final position! On that surface shader you could add anything as a child, like the mentioned default shader with 'snow' settings.
But frost would probably be better seen if the light would be different. I wouldn't add any here.
