What is to be done with a mother who beats her children whenever she gets angry, knowing that she is a Muslim?

You must honor your mother and obey her even if she seems to be in the wrong. You should control yourself and seek refuge in Allah from the accursed Shaytan when she is angry, for the rights of parents are great, and honoring the mother takes precedence over honoring the father. If your mother is harming you or your siblings, act with wisdom and gentleness to avoid being disobedient (aqooq), such as keeping your siblings away from her when she is angry, and seeking help from your father, or your maternal uncles, or wise relatives to overcome this problem.