Where to place/type support for putative relationship

Hello - hoping you can help me muddle this out.

I am researching a friend's line back to the son of a famous figure in the 1800s. The man acknowledged the son in his lifetime and had him baptized, naming the mom and dad. Long story shorter - the family claims to be descended from this son, but there is no record I have found, to substantiate this. (In the time it was not legal for him to marry her because he was a free man of color and she was white) -However the mother of his children took his name as did his two children. Family history claims him, and carried his name down from 1840s to present day.

My question is - I want to include what I have found as support of the relationship but I don't know how/where - under the birth of the children? Attach a note to him and the mother? Add a source? I am sure this may be something simple but I want other researchers to see why I am stating the relationship that not many have researched/found

Thank you for any help.