Meaning: Actually ‘mija’ is not so much slang as a contraction of the words ‘mi’ and ‘hija’: my daughter. However it is often used not only with one’s own daughter but in a friendly way to any girl child.
Hola, mija, ¿cómo estás? Queremos invitarte a Puerto Vallarta para semana santa. Voy a decírselo a tu mamá. Todos nuestros hijos quieren que no acompañes. (Hello, my child, how are you? We want to invite you to Puerto Vallarta for Easter week. I’m going to tell your mother. All of our children want you to come with us.)