My ex my kids mother willfully knew there was abuse going on at her mothers house.
Ignoring my warnings telling her NOT to send them there did anyway and my 10 year old daughter was sexually molested by her step-grandfather.

Would my ex be liable for Criminal negligence because she knowingly sent them to her moms house knowing there was abuse happening.

She was not aware of the sexual abuse at the time but was well aware of the physical and mental abuse that had already gone on. And completely ignored me when I blatantly told her
Not to send the kids there. I at the time had been paying child support and had visitation since day one. I just didn't have any legal papers allowing me to walk in and take them from her.

If Criminal negligence was a factor is there a statute of limitations?

Child abuse such as sexually attacked by the step grandfather is serious. Have criminal charges been filed against this monster yet? You definitely need the help of a good attorney to bring this to the attention of the divorce Judge you had. The judge will definitely not be happy to learn of this and sign the appropriate Orders against her.

It could be but it will be very hard to prove that he KNEW that his father was assaulting the child. You telling him does not make it criminal. Sorry.

On the other hand, this very well might be grounds for changing custody to you if you so choose.

And of course what action did you take before this happened also, why did you not file for an emergancy hearing for

I think the term you are using, "criminal negligence" is an incorrect one. The only crime that comes to mind that might fit your description of what happened might be child endangerment, which would be awfully hard to establish. The easiest way to have this investigated is to talk to your local District Attorneys office. Their job is to investigate and prosecute crimes. My hunch is they are unlikely to be interested in prosecuting.

"Criminal negligence" exists as a concept in criminal law, of course, but it is usually called something else (as I said an example might be child endangerment). The level of negligence needed to establish this element in a criminal case is much higher than would apply in a simple civil negligence case. You might run the question by the criminal law board and see if anyone there agrees. As a family law attorney this is a bit out of my field, but my initial reaction is that it would be pretty hard to establish any sort of criminal liability on your ex's part. Of course, her poor judgment in allowing the child around the step-grandfather might be a reason to request a change of custody and order imposing supervised visitation.