I was molested by my 4years old Nephew

My Aunt came visiting last Sunday and requested we look after her child, our Nephew, since she could not take him along to where she was going to. On this day i also had two of my Niece in the house 3 and 5 years old respectively, my mum, and my younger brother. Am 27 and a huge fan of kids, i always like them around and i play with them alot. After playing with the kids at home on this day, i took a break and sat on the hand rest of our couch. I was putting on a singlet without a bra, my undies and a boxers i stole from my boyfriends house the previous day. As soon as i got carried away concentrating on my phone, my little Nephew climb the couch and rested on my back with his hands around my neck. Soon, he started sliding his hands in my singlet and was touching my boobs. I removed his hand and adjusted myself being that i was uncomfortable. I did not mean it to be anything, i felt he must be mistaken me for his mother because i know male children are always a huge fan of a woman's breast. He did it again and this time i shoved him away. It was more like he was studying me and when he sees am not concentrating he comes close to execute his plans. This time, he came resting on my thigh and was watching a video with me on my phone. Before i gained consciousness of what was happening, his hand was already on my undies, caressing my pussy through the boxers i was wearing. I hit him hard on his hand and drove him away from where i was. The next thing that happened made me realize that this little boy knew exactly what he was doing. I continued with my phone in same position in the living room. After some time my he came back, sat on the couch resting his back behind me. Immediately he put his hand in my boxers touching my ass, my younger brother walked in and in shock this little boy withdrew his hands from my boxers like a thief that doesn't  want to be caught. I was so shocked at the wonders he was performing. I warned my brother to keep an eye on him and not to allow him go anywhere near my nieces. I also told my mum what just happened and she immediately called my aunt's husband to come take his son. I was left to wonder, 'where did he learn it? what if i was not wearing my undies? am sure he would have put his hand in my pussy. And hope he has not been doing same to my nieces? For him to be able to do such with someone my age.