Missing Children's Assistance Act of 2018 (Sec. 2) This bill amends the Missing Children's Assistance Act: to revise the definition of "missing child" to mean an individual under 18 years of age whose whereabouts are unknown to the individual's parent (currently, legal custodian), to specify that a parent includes a legal guardian or an individual who functions as a parent (e.g., a grandparent), to revise the functions and duties of the National Center on Missing and Exploited Children (NCMEC), and to require the NCMEC to make publicly available the annual report on missing children and the incidence of attempted child abductions. (Sec. 3) The bill reauthorizes through FY2023 the following: programs and activities for missing and exploited children, and audit requirements for grant recipients.