Kathy and I wear Snow Board Helmets. They are very light weight, they allow the use of BTH Radio headsets, they have removable ear pads for cold weather and vents that open for warm weather. Plus you can eat and drink very easy, unlike a full face or MX Helmet. Here is a pic of the ones we use and a link to the site.Giro G10MX helmet for snow
When my kids were smaller I had them wear their bicycle helmets because they were lighter and easier on their necks. Now they wear the dirt bike helmets. Luckily havent needed them yet...