Living here in the Midwest, we have mold issues in the bathroom. We want to re-varnish the window, but what will take the mold off the window without damaging the wood?

Some household bleach diluted with a little tap water will do the trick. Too much exposure to any halide, and chlorine is included, will cause goiter. So be certain to wear a respirator and chemical resistant gloves. You can buy both at your local Lowe's or Home Depot, etc.