Don’t worry about the cute little kitten: he has nothing to fear from the pet rat, other than a rather vigorous grooming. He would like to catch hold of the rat’s tail.
Our rats were rough on each other but gentle and playful with us. This rat seems preoccupied with getting down from the chair without having to jump.
I think these two will still be friends after the kitten grows up to be a cat. It’s weird, but that’s domestication for you.