Not being familiar enough with Mystery of History, I can’t compare SCM with MOH. I can help you with where to start with SCM history.

Usually, a high school student needs 1 credit of American history and 1 credit of world history to graduate. Completing the course work as assigned in Early Modern & Epistles and Modern Times & Epistles, Revelation would give your student 1 credit of American history and 1 credit of world history (each course is 1/2 credit of American history and 1/2 credit world history).

I’m not sure what grade levels your other children are but if you start with Matthew through Acts & Ancient Rome your 14 year old, 13 year olds, and 12 year old should all complete the required credits for American history and world history in high school. You will also need to add government and economics courses for high school.

A possible sequence could be:

Matthew through Acts & Ancient Rome (grade 9, grade 8, grade 7, grade 5)

Middle Ages, Renaissance, Reformation & Epistles (grade 10, grade 9, grade 8, grade 6)

Early Modern & Epistles (grade 11, grade 10, grade 9, grade 7)

Modern Times & Epistles, Revelation (grade 12, grade 11, grade 10, grade 8)

Genesis through Deuteronomy & Ancient Egypt (grade 12, grade 11, grade 9)

Joshua through Malachi & Ancient Greece (grade 12, grade 10)

Matthew through Acts & Ancient Rome (grade 11)

Early Modern & Epistles (grade 12)

Your youngest can be given high school credit in 8th grade for completing the books and assignments for grades 7-9 in the lesson plans, so Modern Times would count toward high school credit. I skipped Middle Ages and jumped to Early Modern to give your youngest the other half of the credit for American history and world history. You could also complete Ancient Egypt and Ancient Greece in one year by doubling up assignments, thereby giving your student another year to complete Middle Ages a second time. Your choice; you have some flexibility.