I was  a quiet nerdy 19 y/o girl. I never did anything bad or got into trouble during my years at high school and now i was studying accounting at uni and doing whatever part time work i could get to make some spending money for myself. Tonight i had a babysitting job for a single mother with a 3y/o daughter.
Before my babysitting job i went to my best friends house to chill out for a bit. My best friend Cathy lived with her mum,dad and her 12 y/o sister. Her younger sister  was at school camp for the week and her mum and dad had taken advantage of that and gone away for some alone time. 
We were having a nice cold drink and talking about all the things that had happened at high school and about who was doing what and who was doing who.  Cathy looked at me and said " For as long as ive known you Cindy you have never been in trouble or done anything naughty or even done anything really worth talking about. I love you like a sister Cindy but you really are a nerdy prude. Don't you ever just want to be a bit naughty?"
"I can't believe that you would say that about me Cathy.Just because im quiet and a bit of a nerd doesn't mean that i haven't ever been naughty."
"Oh sure you have been naughty,did you leave your dirty dishes on the sink for someone else to do. No i don't think that you even know what its like to be a naughty girl. Well Cindy why don't we have a naughty bet and find out if you can be naughty or if you really are just a boring prude."
"What kind of a bet Cathy? I've never had anyone know about the naughty stuff that i've done so if you promise to keep it just between us and not tell anyone then i will take the bet and prove to you that i can be a naughty girl."
"Ok Cindy i will give you $200 if you prove that you can be a naughty girl. So Cindy i bet you that wouldn't wear my sisters clothes to your babysitting job tonight.And i mean all of her clothes,her panties and training bra and a cute outfit that i will pick out for you. My mum keeps some nighttime pullups here for my cousin so i think that it would be really naughty of you if you wore one while you are babysitting a toddler that still wears diapers.If you are going to be a naughty girl and take the bet go into the bathroom and shave your pussy completely bald and get yourself a pull up from the bathroom cupboard and put it on.Then come into my sisters room wearing just the pull up and get dressed in the clothes that i have picked out for you. Or you can just be a boring prude and sit here and admit that you arent naughty at all."

Cathy had just totally blown me away and i never thought that she would think about making someone do something so weird or kinky.But i must admit that i was now feeling hornier than i had ever felt before. So i decided to try to do what she had bet me to do,it was something so naughty that i thought that i would feel like a stupid slut. In the bathroom i stripped completely naked and sat on a towel with my legs spread and shaved my pussy until it was completely hairless. Looking at my pussy i was shocked when i realised that i looked alot younger than i actually was. In the bathroom cupboard i found an opened pack of junior girls training pullups and i got myself one out and had a quick look at it. The pull up was pink and had princess fairies all over it with Mummys little potty learner in big pink letters across the front and the back. I didn't think that it would fit me but it pulled up and all the way without any trouble at all,the pull up actually fitted me very well.
Cathy's sister's room was on the other side of the house so i walked through the house wearing just a cute pull up and nothing else. Walking through the house like this was turning me on and i didn't know why. What if her mum and dad came home unexpectedly and caught me dressed like this, what would they do or say.
When i walked into Cathy's sister's room Cathy was sitting on her sisters bed and had a pile of her sister's clothes laid out on the bed. "Oh Cindy that is the cutest diaper and it looks like it actually fits you pretty well. The training bra that i picked out for you is a cute pink lacey disney princess bra so it will match your diaper. And i chose a nice pink disney tracksuit for you to wear so that the baggy pants should hide your cute little diaper."
I put on all of the clothes that Cathy had picked for me and was surprised to find that they all fitted me rather well.Even the training bra was the right size for my little breasts.I never really thought that i had small breasts but now that i saw myself in a training bra i realised that i hardly had any breasts at all. Cathy looked at me and smiled,then she said "I can't believe that you are actually standing there wearing my little 12y/o sisters clothes and my 4 y/o cousins pull up diaper. You really are a naughty girl aren't you. Come and stand in front of me  and hold your hands above your waist because i want to see how hard it is to pull  your tracksuit pants down and expose your pull up."
Stand in front of Cathy i felt like a naughty little girl and i could feel my pussy getting very wet and starting to dampen the pull up. Cathy reached out and gave my pants a quick tug and they dropped to my knees fully exposing my pull up. " Well Cindy you better be very careful around the cute little toddler. It would be so embarrassing if she accidently pulled your pants down and saw your cute pull up. Now over on the desk is a digital camera, bring it over to me and ask me to take lots of naughty pictures of you.And i want you to show off everything including your pull up and training bra."
For the next 30 minutes i did lots of poses and put on a good show for Cathy and her camera. Then she walked me out to my car and told me to have fun babysitting .