We've had pet rats in the past so hope this helps.
Seperate them from mum at 4 weeks as baby rats often become fertile after 5 weeks of age, so make sure the boys and girls are seperate too!! (Rats aren't choosey who their 'mate' is, so boys will mate with mum or sisters)
This site is dedicated to pet rats and has loads of good information
http://www.ratz.co.uk
This is the link to the 'how to tell the sexes apart page!!
http://www.ratz.co.uk/sexing_rats.htm
And don't worry, both mum and babies will be fine...in the wild mum would be almost ready to start the next family by now!!
Good luck