Neighbours cat fighting with my kittens

[FunkinEll]

We have a cat that comes into our garden every day and spends most of the day hanging around.

S/he spends a lot of time chasing/ fighting with my two kittens (7ish months). I'm not sure if they're having fun or if they don't like it. Is there any way to tell if it's harmless or if it's stressing them out?

IME playfighting doesn't tend to be vocal. If they're yowling, hissing or spitting they're not having fun