We read it when my kids were similar ages, some of the chapters were a little long so I often “edited” on the fly skipping parts here and there.
There are a lot of great family books and 1-3rd grade books so if you want to set aside Stories of the Nations and Stories of the World there is still plenty of great reading and books for kids in that grade/age group.
Books are suggestions not mandates use what works for you 🙂 If you continue with SCM they will come back to the time period. In the early grades it is about laying a foundation of enjoying learning, not getting all the facts right now. There is plenty of time to learn 🙂