Is it considered a sin to lie about past sin

First of all I wanna say I really love reading your articles and appreciate your hard work you really help a ton of Muslims through their lives. May Allah Grant you guys jannah Al firdous, inshallah! I wanna ask a question... Let's say there's a 10 years old girl that was on the internet and found a video about sexual and bad stories and watched a couple of videos like that and knew how babies are made. Did she commit a sin by knowing this at such a young age? How does she repent? If it is a sin: what if the time comes where her mother will explain to her how babies are and asks "Do you know how babies are made?" If she said yes then the mother will say how and she'll have to expose her sin, but if she said no does that mean she committed lying?