Touched while babysitting

Recently I was babysitting a few kids that I don’t normally babysit ( it was a favour for my friend who’s their usual babysitter) and she warned me the oldest boy (10 yrs) sometimes can be a little touchy. Tbh I though she was joking and I didn’t really think of it again. As soon as the parents left we were all sitting on the couch ( me and 3 kids ) and started playing minecraft, he moved to sit next to me and started to touch my butt so of course I said no and moved away but it kept happening. Later I was putting them to bed and I had to wait for them to fall asleep ( parents told me to) and he sat down next to me to talk and he put his hand through my sweat pants pocket up my underwear into my “you know” so I said “stop” and tried to move his hand but he kept doing it and he was stronger than me ( I don’t have much upper body strength) so I just elbowed him, minutes later he did it again and again. I babysat them for maybe 5 hours tops and I can name 5 times more he spanked me etc. I under stand little kids do stuff like that “spanking” but grow out of it but my friend and I both agree he’s too old for it really. Do I have an obligation to tell the parents? I feel gross about myself and feel like it’s my fault but I tried to stop him as much as I could. What do I do?