iron (hand|fist) in a velvet glove

a person who appears gentle and kind on the outside, but they are firm, resolute, and even ruthless in their actions

The idiom "an iron hand in a velvet glove" has its origins in the broader history of idiomatic expressions and imagery. It likens a person's demeanor or leadership style to wearing a soft and gentle exterior (the velvet glove) while concealing a firm, strong, or stubborn nature (the iron hand) underneath. This expression is used to convey the concept of being gentle and understanding on the surface while maintaining inner resolve and assertiveness, particularly when making difficult decisions or managing situations effectively.

The iron hand in the velvet glove approach seems to work best with this age group.
The patriarch has always been an iron fist in a velvet glove.Everything was smiles and sunshine in his family, unless you made the mistake of ever going against his wishes.
The new CEO has a reputation for being an iron hand in a velvet glove; she's charming and approachable, but she's also made some tough decisions to turn the company around.
He appears to be a lenient parent, but in reality, he's an iron hand in a velvet glove, setting clear boundaries and expectations for his children.
The diplomat used an iron hand in a velvet glove approach during the negotiations, appearing conciliatory while firmly protecting their country's interests.