I have 2 rats that i purchased from pets at home and they have been fine and are so loveable.

it took a couple of weeks for them to get used to me and you can't force being handled on them as this will result in biting.

your best bet is to just let them come to you. open the cage and stand near it and let them crawl on you and then after a few weeks they will be fine with being handled.

also if you show that you are nervous of them they will feel that fear too and will automatically think there is something to be afraid of ie you! they will then try and bite you in defense
The best way to pick them up is to scoop them up with both hands not to use you hand like a claw. as someone said they are prey animals and this is associated with being picked up by something they will kill you.

the best thing i can suggest is to be patient and let them come to you as they are still babies and they have been taken from one environment and plonked in another so it takes them a little while to settle