take firm but loving action
It has been my experience that if you don't take action and help them from getting into the biting habit it is likely to continue into adulthood. Maybe their biting won't be as bad or as often when they grow up(or maybe worse) but the little kittens have to learn that biting is a 'no no'. I would discourage it as gently as I could (your choice there) and a definite verbal 'no, no." If they learn you are unhappy with the behavior they will get the idea and then you won't have to stop it later when they are adults and its much harder to stop. Nothing worse than having to watch your cat around visitors because she or he might bite them.
A little tough love pays off in the end. Best wishes for success.