This actually happened to me!
So I was babysitting cousin one time (don't know if I would call it babysitting as she was only a couple years younger than me but whatever). My cousin and my aunt would come over and stay at my house for a couple days once or twice a year. One year my mom and my aunt left the house for a couple hours and I had to watch my younger cousin. She and I got along well and we had no problem finding things to talk about. She was barefoot and wearing short shorts and a t-shirt. I was wearing low-cut socks, jeans and a t-shirt. As the day went on we eventually we started playing cops and robbers. I was the cop and she was the robber. Since it was just the two of us, we made it more of a "hide and seek tag" type game but when the cop caught the robber the robber would be the cop's prisoner for five minutes.
We started playing the game and soon enough I caught her. I took her back to my room and commanded her to sit on a chair. She reluctantly sat on the chair. I made her sit on top of her arms so she couldn't move very well. As you all can imagine, I deiced to tickle her. My cousin was quite ticklish, she was especially ticklish on her thighs and luckily she was wearing short shorts. I started by gently stroking her upper thighs with my finger. She giggled and told me stop tickling her but of course she was the robber and I was the cop. I started to pinch and grab her thighs which really made her laugh. I used on hand to hold her legs still and the other to tickle her.
After a while I decided to break out the big guns. The most ticklish spot on her thighs was the outer part. I put my hand on the outer side of her right thigh and started to tickle her. She laughed and tried to get away. I kept tickling her for about two minutes giving her short breaks, just enough time to catch her breath.
Seeing as how I still had three minutes, I decided to tickle her feet for a little while. Her feet were very ticklish but not quite as much as her thighs. I started by tickling her soles. She laughed a lot as I stroked her soles with my fingers. Then I started to tickle her toes and she really started to laugh. After a while she got up out of the chair and started tickling me. She pinched my sides and tickled my tummy causing me to fall on my bed. While I was down she got on top of me and said that she was going to get her revenge. She tied me to the bed, took my socks off and lifted my shirt up.
I let her tickle me since I had tickled her a lot in the past, not just that day. The day before we were swimming in my pool, she was in a bikini and laying out on an inflatable raft. I got on top of her and started blowing raspberries on her tummy and I blew raspberries on her inner thighs while she sat on my shoulders. I had tickled her many times before so there was no harm in letting her get a little revenge.
She started by gently stroking her fingers along my tummy. She positioned her head right over me so her hair brushed along my tummy causing me to giggle even more. She tickled my sides and ribs all while her hair brushed against me. After about thirty or forty-five seconds she laid on top of me with her face right over my tummy. She wrapped her arms around me and took a deep breath. I knew what was coming...... And of course she blew a raspberry right on my tummy. She blew raspberries again and again.
Sure enough after she blew like ten raspberries she went down to my feet. She simply stroked and tickled my feet with her fingers for a while. I laughed quite a bit. After another minute or two of that she said, "I'm gonna try something my mom always did to my feet. It tickled me a lot so I think it will tickle you!"
Before I could say anything she started to nibble and bite my feet. I laughed hysterically and shook my feet. She used her hands to hold my feet still while she continued to bite them. After two more agonizing minutes she let me go. I guess it's what I deserved for tickling her so much.