you are the babysitter not the parent right? first I would not be pushing the child for more information. 9 year olds know more about sex today then when we were kids. its so much more accessible than it was years ago. she might have just happened onto a porn website and saw a couple doing just that, she may be a child who is being sexually abused or she may be a child who happened to walk into the parents bedroom while the parents were getting busy. if she is a child who is being sexually abused you questioning her about it, could taint any possiblity for prosecution or investigation by child protective services.

if this was me I would go to the mother and let the mother know what the child said and find out if the child accidentally walked in on the parents or happened on to a porn website.

if this is is the USA anyone that works (teachers, doctors, daycare, babysitters )with children are considered mandated reporters so I would also be contacting child protective services too. I would be letting them know how the conversation came up and what the child said, letting them know I am not sure if theres abuse going on or not but I would rather report it and find out shes ok than to not report it and find out there was abuse going on.

then I would just leave the rest up to the mother and the professionals at child protective services. working at the crisis center I cannot count how many cases of sex abuse we had to let the perp off because it was a non professional person that continued to question or talk to the child about things, which lead to the child recanting, fabricating more than what was and the defense claimed it was the non professional questioning and talking to the child on sexual topics that created the problem where there was none. one perp even turned around and accused the babysitter of being the one that sexually abused the child after the health exam showed the child had been sexually abused.

I would err on the side of caution and not bother the child about it anymore. you are the babysitter not the mom and not CPS. take it to the mom and CPS and let them take it from here.