What should a couple do if they are unaware that they are related by fosterage, and they have children?

If the matter is merely a doubt about breastfeeding, the original ruling is that the wife is permissible for her husband, and he is not obligated to separate from her. However, if there is strong suspicion, it is more cautious for him to separate from her, based on the Prophet's (peace be upon him) saying to 'Uqbah ibn Al-Harith when a woman informed him that she had breastfed both him and his wife: "How can it be, when it has been said?" But if the husband becomes certain that prohibitive breastfeeding occurred, he must separate from her immediately, because she is considered his sister through suckling, based on his (peace be upon him) saying: "What is prohibited by lineage is prohibited by suckling." As for the children, they are attributed to their father in all circumstances.