There aren't any. It's sort of like donating miles for any other purpose. An airline is going to work with its accountants, tax attorneys, and PR people to design charitable programs. They will bake these into their business plans whether or not you and I provide our miles to "fund" the free flights. The only impact we have by donating is reducing the airline's balance of outstanding miles.

If you have a really good relationship with a local charity that has a use for air travel, you could work with them to donate miles directly - the caveat probably being that you'd have to donate chunks of 25,000 (or more) at a time. If I really wanted to donate miles, this is what I'd do: probably call the children's hospital and start talking to people until I found someone who could put a free ticket to use. But since there's no real infrastructure in place to do this, I'd probably just give 'em a regular unrestricted donation that they could prioritize towards air travel if necessary.

Disclaimer: IANAL. Or an accountant...