As others have said (and I'm just adding in to put more emphasis)... if your rats are eating their babies and they are not first time moms, it's time to diagnose the problem. A healthy rat with little stress and a healthy diet rarely eats her babies. Most of my girls have 10-15+ babies at a time, and rarely do I have a mom that harms them.

Considering the two mysteriously skinny rats and mom's eating their babies, I'd take a closer look at your husbandry. What temperatures are they during the day, night, or how drastic is the difference between the two (dropping from 80 degrees down to 50 at night can be just as stressful as prolonged heat or cold)? What bedding are you using? Food? What type of tubs/cages are they in? Have you checked their water bottles or valves to make sure they are getting enough water? Have their water bottles/buckets/tubing been cleaned recently?