Reunifying Separated Families Act of 2018 This bill directs the Department of Homeland Security (DHS) to reunite each alien child (i.e., under age 18 and no permanent immigration status) who was separated from a parent or legal guardian on or after April 6, 2018, unless a state court or child welfare agency determines that it is in the best interests of the child to remain separated from a parent or legal guardian or there are DHS findings of trafficking or abuse. DHS is subject to a fine of $1,000 per child for each day such child is not reunited with a parent. The bill prohibits an agent or officer of DHS, the Department of Justice, or the Department of Health and Human Services from removing a child from a parent or legal guardian at or near a port of entry or within 100 miles of the border, unless a state court, child welfare agency, or the Chief Patrol Agent or the Area Port Director make findings relating to the best interests of the child or the threat of trafficking or abuse. No such agent or officer may separate a child from a parent or legal guardian solely for the policy goals of deterring individuals from migrating to the United States or for promoting compliance with civil immigration laws.