My heart aches for you and those kids, slimgramma! I don't get the authorities in your state and how they could ignore all this! Maybe they think, ok, kids fall and it's not as if they've being abused or going unfed. But when one puts everything together it spells "endangerment" to me. Please keep bringing it up to CPS, as awkward and aggravating as that may be. Document everything - even take pictures of injuries if you have to. If they finally investigate and your DD and SIL suspect that you GPs reported them, they may be very angry and keep the kids away from you for a while. But I think it may be a worthy sacrifice to make for your GC's safety.
For now, I hope you and the other GPs continue to take the kids as often as possible, as exhausting as I know it can be. You are providing a safe haven for them. Bless you (and the other GPs)!