You didn't say which airline or airlines you have these points with, but most of them have a section of their Web site with this information as it applies to that airline's miles - which is, after all, the only information that matters. For example, AA's discussion of donating to the Miles for Kids program is here.

I know you asked specifically about donating, but you can also often use small numbers of "orphan" miles to purchase magazine subscriptions. These, in turn, can be donated to someplace like a senior center.