3:47 PM PT -- A rep for Chris tells TMZ ... Shar has every right to respond however she'd like, but Chris and his family are choosing to settle this in court and not in the media or on TV.

Shar Jackson is apoplectic that Chris Massey accused her of abusing his 2-year-old daughter (Shar's granddaughter) ... saying her very public history as a parent is more than proof she'd never hurt a child.

Shar bristled at Chris' claims she hit his daughter, Mariah, last month and left a razor blade cut on her forehead. Shar -- who also had 2 kids by Kevin Federline -- says the claims are boldfaced lies and she's now got her own plan of attack ... against Chris.

As for the alleged razor blade injury to Mariah, Shar says the girl accidentally cut herself during a Facetime between her mom (Shar's daughter) and Chris back in May. Shar says she wasn't even there.