THis is a terrible situation to be, but right now, it would seem that you have done the best thing right now, as CPS is involved. You can certainly seek to amend custody based on what the children are telling you. One concern would be if CPS does not take action on the same evidence, it might be a pre-curser to what a judge would do with the same evidence.
That said, I would almost always advocate that if you truly believe these children are being abused, you owe it to them to take whatever action might help them, even if it turns out to have been futile. For that reason, the one step that I can think of taking if CPS is not acting is to immediately contact a family law attorney and find out how to get an emergency motion to change custody on the docket as quickly as possible.
In the end, you may have a hard time if CPS will not act, as whatever is preventing them from acting will likely hamper your ability to prevail with the court. But this issue is too important to make a decision based on the likely outcome. If you do not file, the children will certainly be left in that home, subjected to continued abuse. If you act and lose, that is on the courts and CPS and let them try to sleep at night. IF you act, even if it is likely that you will lose, it is still possible that you will prevail. That possibility, no matter how small, is worth it if it means stopping this child abuse.