If you add a source to a person’s page and “add others who appear on the record” and select to add a child, the child will be added as if the child’s other parent is unknown (see screenshot below). I think you will have to add a prompt for whether this child is a child of a known spouse or an unknown spouse. Alternately, you might have to disallow adding a child in this way.
BTW: This seems like a perfectly reasonable way to add a family from a census record (head, spouse, children). In this case, the spouse might be a new record and would have to be included in the prompt.