A question about nibbling babies.

Ok I have thirteen babies and not all but some of them are pretty nippy. What are some things that I can do to curve that? they are about three weeks right now. Very cute friendly and licky but at the same time some like to nibble a little to hard. Not enough to draw blood but enough to where u feel it and want to jerk away but don't. I want them to be safe and happy and loving before they find their forever homes. :lol: