It is not complicated or difficult.
You just add the Child to the Mother, who will NOT have a Spouse, just like adding ANY Child (individual/person) into "Family Tree" of "FamilySearch".
There are many individuals/persons that exist in "Family Tree" of "FamilySearch" just like that.
There are many "Illegitimate" Children, born 'Out of Wedlock', that exist in "Family Tree" of "FamilySearch".
And, conversely, there are also many individuals/persons that exist in "Family Tree" of "FamilySearch" with ONLY a Father; and, NO Mother - many of these may have been created from a "Marriage" record where only the Father is recorded.
Here is a "Knowledge Article" in "FamilySearch":
How do I add a child to Family Tree?
Where it states:
• You can link them to the biological parents individually, or as a couple. Use the above instructions to attach the child to their biological parents.
• Do not add any marriage information for the couple, since they never married, but relevant information can be included in a note or in the individual's Life Sketch.
• Currently, Family Tree provides five relationship types between parents and children: Adopted, Biological, Guardianship, Foster, and Step. The system assumes a biological relationship. You can specify a different relationship type for multiple sets of parents.
I hope this helps.
ps: Whereas, if you are referencing "Temple" Work for such Children (individuals/persons); then, that is another matter ...
Thank you , very helpful