A firm hand
Definition 1 of 2
Pro Tip 1/3Authority Context
Used when someone enforces rules or standards, typically by someone in charge. 
The new teacher managed the classroom with a firm hand to maintain discipline.
Pro Tip 2/3Not Harsh
'A firm hand' suggests control without being overly harsh or cruel. 
She ruled her team with a firm hand, but always with kindness.
Pro Tip 3/3Parental Guidance
Often used to describe a style of parenting that's disciplined but fair. 
With a firm hand, the father set clear boundaries for his children.