I see message about block site visited - How a child can visit blocked website?

We have youtube.com as a blocked website. However, we see that child has visited blocked website and upon viewing details on NOF we see following. How is that possible or how child is figuring out to bypass parental control. We are using windows 7 OS Web Activity for tc.v17.cache1.c.youtube.com http://tc.v17.cache1.c.youtube.com/crossdomain.xml