If a kaafir becomes Muslim, what is the ruling on his illegitimate children?
Question

                Someone is asking about a serious problem which faces most of those who enter Islam, which is that before they became Muslim they used to commit zina (fornication) and that resulted in many children. Are these children who were produced in this manner entitled to maintenance from their fathers, knowing that there was no marriage contract between them and their mothers?.
Praise be to Allah.
    In the case of those who had intercourse when they were 
    kaafirs, if they believed that this intercourse came under a marriage 
    contact that they regarded as valid – even though it is invalid according to 
    sharee’ah – then the children belong to the man. For example, when he was a 
    kaafir, a man proposed to a woman that she should be his wife and she 
    agreed, and they thought that this was a marriage contract, then the man and 
    woman become Muslim. In this case we say that their marriage contract is 
    valid and does not need to be renewed, and the children that they have had 
    together belong to them, unless the wife is not permissible for the husband 
    when they become Muslims, such as if he was a Magian and married his sister 
    – because the Magians regard marriage to mahrams as permissible. If he 
    married his sister when he was a kaafir then he became Muslim, and she also 
    became Muslim, then they must be separated, because the woman is not 
    permissible for the man. So we say to those who are mentioned: If you 
    believe that your relationship with this woman was marriage, then it is not 
    zina, and the children belong to you. If you believe that it was zina, then 
    if you accepted these children as yours when you were kaafirs, then they are 
    also your children as Muslims, so long as there is no one else claming them 
    as his. If they did not accept them as their own, then they cannot be their 
    children. 

    With regard to maintenance, if we rule that they are their 
    children, then they are obliged to spend on them, and if we rule otherwise 
    then they do not have to spend on them. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him). 

    Liqaa’aat al-Baab il-Maftooh 
    (3/385-386).