Kids aren't usually that...stabby. What did Qyburn do to them? They became Ramsay Bolton's ready-made heir material in maybe a few weeks. What do you suspect?
Book Varys is more overtly ruthless than show Varys, so it wouldn't have worked for him to kill Pycelle himself-especially since the moon tea Marg accusations never happened and Kevan got blown up in the sept. In this respect at least, Anton Lesser's Qyburn fits the assassin role nicely, but he got 30 or so kids to brutally stab people! I've heard people call him Westerosi Josef Mengele, he's a scientist and a convincing manipulator, so it's certainly possible he drugged or brainwashed them, but I sort of wondered if Varys still secretly controlled the little birds, found out the Pycelle assassination plot and the wildfire stuff and let it happen so that more people hate the queen, only then to resume control in Westeros and have them turn on Cersei's new plotting buddy.