Party’s or Parties or Parties?

In a legal document, I am referred to as a “party” in some instances. The legal secretary wrote, “...Order modifying child support using the parties’ actual income...”

Is this correct?

Yes, because the context indicates that the plural possessive of party is required.

Party's is the singular possessive.
Parties is the plural. For example, "Both parties agree ..."
Parties' is the plural possessive.