teke,
I can only share my experience and thoughts. My newborn granddaughter was in an abusive situation with my son. I arranged a safe place for his girlfriend and bought her a ticket out of state so she could take my granddaughter to her mothers and live there away from my son. Calling CPS would have been a last resort and fortunately the plan we had turned out well. The only down side is I've never seen my granddaughter since that time. It took me some time to make that decision because I was putting pieces of a puzzle together too. I lived in a different state and didn't have first hand information. I finally got enough information to come up with a plan.
If I've read your post right it sounds like you have the kids now and can take some time to explore your options. I know that there are lots of support groups for grandparents. Maybe you can find some local resources and support to work this through and come up with a good solution.
I'll keep you all in my prayers.
Hugs,
MG