Is the marriage considered void and her son illegitimate if she continues in the marriage after engaging in sexual intercourse during menstruation, doing so intentionally, and must she seek a divorce or inform her husband despite fearing the consequences, and how can she repent from this sin?

You sinned by enabling your husband to have intercourse with you during menstruation, and this is forbidden by the explicit texts of the Quran and Sunnah. You did well by repenting to Allah, for His mercy is vast for those who repent. Do not inform your husband about the matter, as it is obligatory to conceal one's own sins. There is no need to inform him about the loss of virginity to someone who did not stipulate it. Your deception and fraud do not affect the validity of the marriage, for the marriage is valid and your son is legitimate. Be at ease and do not succumb to whispers.