to start to control something or someone more carefully.
She has tightened the reins on her younger sons in an effort to curb their wild behaviour before it's too late.
(often + on)
tighten the reins
To begin doing something more carefully or cautiously; to regain or tighten control of someone or something. This thesis you're planning is becoming unfocused. I think you should tighten the reins a bit.I wish those parents would tighten the rein on their kids, the little devils are tearing the place apart!