One time through Early Modern and Modern Times is fine. The most important thing if you live in the US is to complete Early Modern and Modern Times in the high school years to give your student American and World history credits for a college transcript. If you would like your older student to read some of the books scheduled for the younger grades in Early Modern and Modern Times, then you could assign those to be read alongside the Ancient History lesson plans or encourage your child to read those during free reading time.