You think Tywin would've been powerless in the face of the Sparrows?
lol. Those insufferable zealots actually make me miss Joffrey.
He would be if they had imprisoned Jaime and Cersei.
The Sparrows have leverage on House Tyrell at the moment, mainly because of Cersei's fatal combination of hubris and stupidity, and because Tommen is a weak king. An army from the Reach commanded by Randall Tarly (one of Mace Tyrell's bannermen) could take King's Landing and crush the sparrows, but it would probably mean Loras and Margaery would be murdered by their captors before the city fell.
Tommen should summon the High Sparrow for a meeting, ostensibly to negotiate, but when the High Sparrow makes his appearance, have the King's Guard seize him and slay anyone who gets in the way. Simultaneously send a raven to Randall Tarly and have him start to move an army into position to attack King's Landing. Negotiate a prisoner exchange with the sparrows, exchanging the High Sparrow for Cersei, Margaery, and Loras. After the exchange, deliver an ultimatum for the sparrows to depart the city. If they refuse, attack the city with Tarly's army and annihilate them.
Tommen however is a weak king, and would rather sit in his room and sulk than take any aggressive action.