I haven't had the chance to tickle anyone in a very long time but when I did I used a kitchen brush. The ones with the wooden handle and the light whitish tan colored bristles. Looks like a paintbrush but is used for applying marinades on food. Anyway it was either a 1.5 inch or 2 inch width. When the bristles were horizontal to the foot it was only mildly effective but when I turned it so the bristles were vertical and did long deep strokes up and down the sole while getting in between toes at the top of the stroke I got a great reaction. That reaction where the lee starts breathing heavier and heavier before breaking out into giggles. it was really hot. Cue tips also elicit a nice response in my experience.
Since this is the topic, I bought some woolite the other dat that came with a soft green fabric brush attached to it. If your looking for new tools you might give it a try. It looks and feels like it could do some work on a sensitive pair of feet.