take the bit in one's mouth

take the bit in one's mouth also take the bit in one's teeth  {adv. phr.}

To have your own way; take charge of things; take control of something.

When Mary wanted something, she was likely to take the bit in her teeth and her parents could do nothing with her.

Compare:

TAKE THE BULL BY THE HORNS,

TAKE THE LAW INTO ONE'S OWN HANDS.