Presumably you simply mean a child under age 12? In Southwest parlance a Young Traveler (age 12-17) is the category above Unaccompanied Minor (5-11), and Young Travelers do not need to be linked to any other reservation as they can travel by themselves. For children under age 12, their reservation must be linked to an adult (age 18+), otherwise Southwest will consider them an Unaccompanied Minor, which triggers the UM fee ($50 o/w).