Well, I don’t know what is covered in Story of the World 1&2…..
But here is the summary of the modules, and then you can select one, look at the details, and even download a generous sample! http://simplycharlottemason.com/planning/scmguide/history-modules-overview/
In short
Module 1 – Ancient Egypt
Module 2 – Ancient Greece
Module 3 – Ancient Rome
Module 4 – Middle Ages / Renaisannce
Module 5 – Early Modern
Module 6 – Modern to present.
Whichever module you pick, you would put all your kids in at the same point.
You would buy the Family Books, and do those together. Assuming your kids are at about grade level skills, you would get the grade 4-6 books for your 4th grader to read on their own, and the grade 1-3 books to read to your 1-2 grader, and you could have your 4yo listen in on any of the grade 1-3 books they are interested in.
With the Guide, it telss you for each day of school which chapters in which books to read. They are very well laid out.
hope this helps!