Is defending a child who is being beaten and humiliated by their grandmother considered disobedience to parents (ʿuqūq al-wālidayn)? And what is the religiously prescribed duty towards the mother in such cases?

If a mother severely beats a child, she is oppressive, and she should be advised gently. If she does not stop, then whoever can prevent her, such as her husband or father, must be addressed. Preventing her from oppression is not disobedience but rather an act of righteousness towards her.