can another parent sue the parents of a child calling thier child a whore and/or slut?

I think you are going to have a difficult time getting an attorney to file such a lawsuit because there is no real financial upside to getting involved in that kind of litigation for an attorney. A more practical route might be to contact the local police and find out if you can prosecute a criminal complaint against the offenders.  Please note that by attempting to answer your question, I am not acting as your attorney. I will do nothing further to protect or preserve your interests in the absence of any additional discussion with you about this matter. John Ratkowitz, Esq. Starr, Gern, Davison & Rubin, P.C. 105 Eisenhower Parkway Roseland, NJ 07068 Office: (973) 830-8441 Cell: (732) 616-6278 Fax: (973) 226-0031 Email: jratkowitz@starrgern.com  Skype: john_ratkowitz Web: www.starrgern.com.