We have sucess - arsenicum was what my baby needed (thanks, Snoopy). Clinging from fear was the key, whereas I had assumed that it was clinging from teething or some other discomfort. She plays happily and doesn't scream anymore when that little boy comes near her. Now that poor little boy is crying all the time because my daughter is picking on him again (back to her wicked ways, again). Oh well - I'm sure this is part of her constitutional, which I still haven't figured that one out yet but that is not urgent.
Again, thank you everyone for your input - I learn so much from the people who post on this bulletin board.