I would like to ask a question about a non-Muslim girl who married a non-Muslim man but who has since converted to Islam. Can she stay married to her husband even though he is still a non-Muslim? Kindly note that the couple have children.

It is prohibited for a Muslim wife to have physical relations with a non-Muslim husband based on the words of God Almighty, "They are not lawful [wives] for them, nor are they lawful [husbands] for them." (Quran 60:10).

A Muslim wife is not to seek a divorce except until she informs the judge about her Islam and the judge will inform the husband. The judge will then invite the husband to Islam. If the husband accepts Islam, their marital life will continue based on the previously conducted marriage contract; otherwise, the judge will issue a ruling to separate them. This separation is based on irrevocable divorce. The woman's waiting period will start on the day the judge issues the ruling.
And God Almighty knows best.