Are you looking for advice? If so, watch for damage to the skin but otherwise it's fine. I had a litter of orphaned kittens and they did this to each other. I was warned that the only thing I needed to be concerned about was if they were sucking on genitals because, apparently, it can cause issues later and, goodness knows, male cats already have a high enough risk of blockages.
#PetBehaviorandTraining