grammatical number - How do you make a word like "parent(s)" possessive?

"Please submit your and your parent(s)' federal tax returns." Is the possessive of "parent(s)'" correctly formatted in that sentence? I know the apostrophe comes before the "s" for just parent singular, and it would come after the "s" for parents plural, but what if I don't know if it is one or two parents, and therefore want to use the (s)?