About 5 or 6 years ago (I think), so sometime between 2012 and 2015, I caught most of the first episode of a dark new TV drama series. The episode began with men from an unknown organisation breaking into a family home to kill a child. As far as I recall, that child (or another) spoke at one point as if an adult in a child's body, making threats and hinting at old, secret knowledge. The episode seemed to suggest the existence of a secret group of people who were living on by serially taking over the lives of children - at least, that was my guess.
I wasn't able to see the whole episode because of various interruptions. I assumed I'd be able to catch up with it later but never found the time and that was a busy and disruptive time in my life. By the time I remembered the show, I had forgotten the title.
I'm pretty sure it was meant to be a one-off mini-series.