Answered by Shaykh Faraz Rabbani
Can parents backbite about their children (e.g. if one parents is speaking ill about another one of his/her children to another child with the purpose of criticizing). Some people say that parents cannot backbite about their own children.
Words spoken out of legitimate and reasonable concern are not considered ghiba. So criticism made in this manner would not be ghiba, but just speaking ill of one’s children for no purpose other than venting one’s frustration is at least blameworthy, if not outright impermissible.
The general principle of speech was lucidly expounded by our beloved Prophet (Allah bless him and give him peace):
“Whoever believes in Allah and the Last Day, let them say the good or keep silent.”
The “good” here is anything of worldly or next worldly benefit.