Etch A Sketch. Leslie of A Room Somewhere recently posted this easier-than-you-think project from the defunct (and dearly missed) Budget Living magazine.
We love the face silhouette used here and you could do basic ones of your children or choose any shape you like as long as it's big enough to make the mirror usable (unless you want the mirror to be purely decorative). You could also try this idea on a small window or part of a window.
You'll need a mirror, contact paper, etching cream and a brush. See the quick instructions here.
If you try this - send us a photo or upload it to Flickr (tagged apartmenttherapynursery).