Volunteer Match (www.volunteermatch.com) is a great way to find all types of opportunities - you can search by interest or geographic area.
Plus I know of three great nonprofits that work with kids. Two - The Service Board and Burton Chill - teach at-risk kids to snowboard, plus The Service Board also adds community projects to the mix.
And Outdoors for All works with disabled kids and adults. Snow sports are their primary focus, but they have adaptive sports options year round.
All three need coaches plus other volunteers.
Get information about The Service Board at http://www.theserviceboard.org/get_involved.html or visit the Outdoors for All Web site at http://skiforall.org (they used to be Skiforall) for information about that program. I only have national-level contacts at Burton Chill, but they are advertising on VolunteerMatch for local needs.
If you want more information about any of the programs, contact me. I've worked with The Service Board and Outdoors for All and probably will work with Chill too this winter.