Is there an organization who repackages old LEGO bricks into sets to give to underprivileged children?
Another option is to locate a nearby BrickLink vendor and request them to sell the pieces and donate the proceeds to charity. So far as I know, this is the best way to donate, as the pieces are sold for their appropriate value and the money can be donated as you wished. You may consider donating the pieces to Creations for Charity as well.

If you really want to donate parts, the best way would be to use the money from other suggestions to purchase general purpose block sets, and donate those.