I play it by ear a great deal. With kids as young as yours, you can be quite flexible. At least, it has worked that way for us. There are as many ways to schedule things as there are families, and if your first year is anything like ours, you’ll change things up several times!
By September (after all the summer birthdays) my kids will be 7, 5, and 3. We also have a 4yo girl and 15 month old twins that I watch everyday. We usually do most school in the afternoons, when the twins are napping. Sometimes my DD(7) and DS(5) will actually ask to do something in the morning, which I try to work with, but mornings are pretty chaotic, so that doesn’t always work. We do try to fit science and nature study in during the mornings, since those are more active and don’t work when we have to be at home and quiet during nap time.
After lunch, I try to start with our family studies, since I want the preschoolers to be part of at least our Devotional and Scripture Memory work before their attention wanders. Then we usually do our weekly type activities (poetry, artist, etc.), with the little ones allowed to drift away to play as they choose.
Then I decide who to work with based on whether DS seems to be still focused or needing a break (or has already left). DS and I just do a chapter from Life of Fred or play a math game with DD, work on printing numbers or other fine motor activity, and read a chapter from a literature book (he reads a page, I read a page). While he is doing that, DD either just works on a project (drawing or some other craft, she always has something she is creating), or does some computer work (Spanish or Typing Instructor).
Then DD and I do the rest of her work. I let DD choose what order to do things. She almost always chooses books first so she can continue her project while she listens and narrates. Then she’ll do Life of Fred and Copywork last. This isn’t breaking up similar tasks the way CM recommends, but it works for her. She focuses much better when she has something to do with her hands as she listens, and it frustrates her to have to leave and come back to her project.
If DS is not focused after the family work, DD and I will work first while he goes to play with the other kids, then I’ll bring him back.
All of the above is never quite as ideal as it sounds, as we have frequent interruptions from younger children, but it gives you the basic idea of what we do.