Banquet table situation - was I wrong to move?

You weren't wrong. This woman was embarred, and instead of addressing the actual issue, that her family's behaviour was so bad it drove you away, she chose to try to shift the blame onto you. Her granddaughter may have mental issues which make her act out in a public setting, but the appropriate response (on the adult's part) would be to take her from the room until she settles down, not resort to juvenile behaviour herself.