I hope someone will chime in as to how they manage that...
I have similarly aged children - 10, 9, 7, 5, & 3. (And their grades are roughly 5th, 4th, 2nd, K, and Pre-k). I will tell you what I manage to do. I have not pulled off rowing multiple levels, so this may not interest you. I just cannot pull my brain in that many directions.
We are all rowing together, and we're in volume 4. This seems to offer plenty of challenge for my oldest daughters, and not too much for my 7yo. My 5 & 3yo tag-along with "younger" go-along books, participate in geography, art, some math, and cooking. For my kids I add copy work for my oldest 3, and some light grammar and spelling for them as well, also math. My little girls work on phonics and math games. We often include a chapter book go-along (to be read together, or individually if it's an easy-reader for my 7yo, or get some books that appeal to their interests to be read alone and shared, etc.)
This may not sound like much, but it's been very satisfying for us. And it's sustainable. I just cannot sustain all the planning each week/month of 3 different levels. This just may be a personal limitation. It may be different if we'd rowed these books before, but they're new to all of us. We're enjoying doing one study, as a family, and it's given us the flexibility to follow some interests since I don't have to move on to something else with another set of kids, ya know?
Let me know if you have any more questions. Have fun with. your kids!
Mindy - wife to Colin since '03, mommy to FIVE fabulous kids dd10, dd9, ds7, dd5, dd3