Very nice story. I truly enjoyed it.
One tiny suggestion, and this of course, is optional. Everyone doesnt do it this way, but, sometimes, authors, including myself, write the laughter into the dialogue, at least a bit, so we can feel what the ticklee character is feeling. A couple of well placed "HAHAHAS" here or there, can sometimes serve to show the reader just how tortured the character feels.
As I said before, fine story, and the other thing was just a suggestion.
Mitch