Slander is a type of defamation. It is a civil issue and you would file a lawsuit (rather than reporting it). You would have to prove that he said these things to another and that they were untrue. Unless the slander is on certain topics you must also prove damages (that the slander cost you money, etc.)
When you write about harassment in a "degree" and "report slander", it suggests to me that you are looking for help in criminal law. If you believe you are the victim of a crime you should report it to the police (though they may inform you this is a civil matter).
Finally, what I really think you are getting at is that you want to make him stop. For that you would go to court to get a restraining order. If you can't prove the harassment you could be liable for attorney's fees, and there are other consequences as well in family law if you allege abuse and fail to substantiate the allegation.
Regardless, you have selected a challenge for yourself having children with two different people with whom you do not get along. Good luck in all of your endeavors!
In family court the father will be ordered not to say such things to your children, but it is not easy to enforce. The Judge has a lot of discretion and could use it against him by giving you more time with the kids and increasing the child support dad owes you.
As for slander, that is not an issue I'm trained to address.