“Pet her gently, OK?”
You're visiting a friend's house. They have a small cat. Your son wants to pet the cat, but he pets her a little too hard. You want him to be careful.
Pet her gently, OK?
"Petting" an animal means rubbing its fur.
You can describe the way that someone pets an animal as:
pet it roughly
pet it gently / softly
A very gentle and kind-sounding way to tell a young child what to do is to finish your instructions with "OK":
Cut it carefully, OK?
Put your toys back in the box, OK?
With an older child or adult, you should be careful because it might seem like you think of them as a child if you give them instructions in this way.