Interesting question... I was playing around with some ideas and made a few (bad) sketches.
At first I thought about basing a graphic on an exclamation point, to convey the excitement of a tickle. Maybe you could make it look like the top part of it is tickling the dot at the bottom, somehow... maybe the dot could have a smile on it, and there could be little marks around it to indicate motion, or sweat drops coming off it... something like this:
View attachment 348193
Then I thought maybe the top part could look more like a finger coming down to tickle the dot. Also, maybe the dot could look like the planet Earth (although that might be a bit complicated)
View attachment 348194
Then I started to redraw the top part, and I realized that a simple drawing of a finger with a fingernail, also could kind of look like a laughing face, especially if you add a few lines at the top that could either be eyes and eyebrows for a face, or... IDK... knuckle wrinkles or hairs for a tickling finger or a tickled toe!
View attachment 348195
Just a few ideas off the top of my head, maybe you can make something good out of them...