My daughter, my girlfriend's kids and my nieces are doing something they shouldn't do and I'm going to use a strict tone of voice when telling them not to.
Do you find what they're doing to be dangerous?
Yes, they're going to get hurt if they keep doing it.
It's a good idea to act strict when you tell the kids that what they're doing is dangerous and they could get hurt. The kids will definitely respond better if they know that you're serious, and that you're telling them in order to keep them safe.
I would never hurt them, but I want them to understand that I'm serious.
As the adult it's your responsibility to keep children in your care safe and sometimes that means being serious and firm with your voice when telling them to stop something that could hurt them. You're doing the right thing and only looking out for their best interest.