Are the children sinful for abandoning their father who deserted them a long time ago, has no regard for them, and claims knowledge of sorcery, especially after their mother's dying wish that he not enter her house, and the children's embarrassment over their father's actions?

The mother's will is not binding, and it is not permissible to abandon the father. Rather, honoring him and accompanying him with kindness is obligatory, no matter his condition, and severing ties with parents is among the gravest sins. What is obligatory is to maintain ties with the father and to repent from abandoning him, and to continue advising him, enjoining good and forbidding evil with gentleness and politeness. If you advise him, Allah will not hold you accountable for his actions, for "no bearer of burdens will bear the burden of another."