Is it permissible to contract an urfi marriage with a Christian girl, knowing that she might be exposed to danger, especially since my current wife is asking for a divorce and I fear for my children?

Marriage to a girl without the knowledge of her guardian is invalid. It is permissible to marry her through her guardian from among the people of her religion (her father, then her grandfather, then her brother, then her uncle). A condition for the permissibility of marriage to a woman of the People of the Book is that she be chaste. If you are in need of marriage and are able to provide for it and observe justice between your wives, it is preferable to marry a Muslim woman of religion due to the risks involved in marrying women from the People of the Book.