Yes it's illegal. Allowing a child to engage in sexual contact with an adult, regardless if the child started it or not, is against the law.
It'd be just like walking in on a child drinking from a bottle of alcohol and saying "Well, I didn't give it to her so it's not illegal for me to let her drink."
It sends a message that you're condoning the activity.
But I wouldn't worry about it too much. If she started it, it's not likely she's going to tell on you.