Is it considered disobedience, warranting sin, if children refuse to go with their mother to file a lawsuit for alimony against their father, who has not provided for them for 20 years, especially since the mother is angry and displeased with their stance?

A father must spend on his young children who have no money. This is his sole duty if he is wealthy. The obligation of supporting relatives lapses with the passage of time. As for demanding future maintenance, there is no harm in it. It is not considered an act of disobedience if the father refrains from providing the obligatory maintenance, and the son has the right to demand it from him and compel the father to provide it. If the lawsuit is from the mother, demanding what she has spent, then you must testify with truth and justice, and the judgment thereafter rests with the judge who will decide whether the mother is entitled to what she spent.