All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
We underlined previously that it is impermissible for Muslims to attend baptisms and other rituals that involve unbelief. Please refer to Fatwa 90151.
However, if this woman did so out of ignorance of the law, then she bears no sin for it, especially since she has recently converted to Islam. It should be noted that isolated act of attending the baptism does not constitute an act of disbelief that takes the Muslim out of the fold of Islam; she is still considered a Muslim and she does not need to repeat the testimony of faith (Shahaadah). She should beware of doing such a thing again in the future and should know that it is impermissible for her to attend such a ceremony whether her husband gave her permission or not. If she had gone out of her house without her husband's permission, then she has committed a misdeed and has to repent for that.
There is no harm in concealing her Islam if there is a need for it; otherwise, it is better for her to announce her Islam. Verily, concealing one's Islam may incur some evils like failing to perform some religious rites. In any case, it is impermissible for her to neglect any of the religious obligations, especially the prayer.
We advise her to supplicate Allaah to guide her children to the Straight Path and to strive to have a good influence on them to the best of her ability, especially since once they reach the age of eighteen, it will be possible for them to live independently and outside of the influence of their father or any other person.
Please refer to Fataawa 81979, 88906 and 88051 underlining that Islam is the true religion of Allaah and Fataawa 8210, 10326 and 90449 proving the falsehood of the present-day Christianity.
Allaah Knows best.