Xfinity Flex Streaming Box Parental Controls

I find some of the advertisements inappropriate for my children that come up on the infinity flex box. If I don't let my children watch TV content y7 or higher or movie pg13 content or higher, I would like to block advertisements for programs outside the content I would allow them to watch (they're 5 and 2). Is there not a way to block this? I've searched the parental controls option and it only allows me to block apps, but not the xfinity advertisements themselves.