I'm torn between Tea for Two and Sweet Charity
I like Sweet Charity, but wonder if people will think it sells sweets? I agree it shouldn't mention the charity aspect, although you could have a board inside telling people where the profits go. You could sell vouchers for a special Afternoon Tea, as a present idea. (nice treat and supporting local charity)
You want visions of cream teas, big chunks of home made cake and piping hot tea and coffee.
I think a lot of people do like an old fashioned name, perhaps with the name Tearoom. Or - Polly put the kettle on, or Pollys' Tearoom.
Good luck with your venture, how exciting. Wish I lived near, I'd support you regularly!