Be careful about recording. It is not legal in some states to record some one with out their knowledge. If it isn't legal in your state then you can't use it. :( Stupid I know but that is the law.
Your best bet is to get a counselor for your children. Then if there are signs of abuse the counselor can testify in court.
Good luck

I agree with Lisa on the recording issue. Most states will put you in jail for invasion of privacy! Lisa also made a good point about taking the kids to counseling, that is perfectly legal. Although, some counselors may charge a fee to appear in court for you (I remember that from going to marriage counseling). If the counselor determines that their mother is abusing the kids, he or she will report it to social services. If you cant afford to take them to a therapist, or dont have insurance coverage, have them talk to the school psychiatrist, they will also contact social services if needed. If she is not physically abusing the children, I dont know if they will do anything though.