In that meeting with Kevan and Olenna the two Lannister twins demand a violent approach to resolve the "High Sparrow problem": though King Tommen has explicitly forbidden that violence should be employed against the sparrows - for fear for the life of Queen Margaery - , this is exactly what Cersei proposes.
To bypass Tommen's order, the attack on the Great Sept shall not be performed by Lannister forces, but by a Tyrell army. After some argueing and bickering, all of the four agree. And when this happens, Cersei's expressions suggest a whole bunch of emotions - not only relief, but clear hints to something much much darker behind it.
What gives this scene a special undertone of danger and doom, is above all the musical theme that is playing in the background:
THE RAINS OF CASTAMERE.
We can be very sure that this has a meaning - no way it has slipped in there by accident. This theme signals doom and destruction to the enemies of House Lannister, and in this case it seems to announce the Castamerization of House Tyrell.
First of all, there is Lady Olenna's patronizing remark towards Cersei at the beginning of the conversation:
This reminds of what the Lord of Castamere said: "And who are you, the proud lord said, that I must bow so low?"
This is what Cersei says to Kevan, obviously talking about his son Lancel. But I get the feeling that in her heart and mind she is at the same time speaking about her dead son Joffrey. When she says this, she moves to the side of Jaime, who is slighty nodding in silent confirmation.
So in a second meaning, "they" would refer to the Tyrells who organized Joffrey's death (together with Littlefinger); it could very well be that this information was the secret the High Sparrow told Tommen (which would also explain Tommen's shaken state of mind and tearful expression when talking about this to his mother).
By the way, the Rains of Castamere theme DOES start before the council meeting, right during the conversation between Cersei and Tommen...
And then Cersei continues, still talking to Kevan:
"[...] stand aside and let the people who took him from you be destroyed",
after which the camera instantly switches to Olanna Tyrell, who's averting her eyes...
All of this give a very strong impression that Cersei has another motivation and another plan besides what she states in the meeting.
As Jaime said in Episode 1:
"Everything they've taken from us, we're going to take back and more."