My rat likes to nip my fingers, help!?

About a week and a half ago I brought home 2 female rats and they are adorable as can be. I am trying to handle them more now that they have gotten comfortable with their cage and everything. I will put my hand in the cage and the peach colored one will come out of her house sniffing. Once I thought she was going to climb onto my hand because she was sniffing it and had 2 paws on it but then she pulled back and bit me. She didn't bite really hard but hard enough to startle me. The black one has never nipped and just seems scared. You kind of have to chase them down in their cage to handle them. The peach one is the only one that will come to you but now I'm afraid she is just going to bite me. She has nipped me twice now. We have a toddler and I don't want her nipping at him too. Does anyone know of anything I can do to stop her from nipping and help with getting them comfortable enough to be held and played with? I don't think I did anything wrong when she nipped me, I didn't pursue her, she came to me and I made no swift actions to picking her up or anything. Please help! I want to be able to enjoy our new little furry family members not fear them nipping at me or possibly even biting me or my son or fiance.