I really enjoyed this post over at Raising Arrows this morning. Even though we only have four children right now and only three of them are technically "schooling," we still get this same question.
I like to think of our schooling time as very "one-room-schoolhouse" style. Even though the littles may not be ready for some of the more mature history discussions, they are still in the room and part of the story and discussion. They also may not benefit from some of the more advanced math and science concepts but the exposure is wonderful.
My new favorite example of why and how this works happened just a few weeks ago. Our youngest (3) was playing with some Lauri toys recommended in the My Father's World toddler and preschool packs, the two oldest were working independently and I was reviewing flash cards with the Kindergartner. The baby wondered over and started mimicking the Kindergartner as she reviewed her numbers. When we came to the flashcard number 3, he ran back to his Lauri toys, grabbed a foam 3 and returned all smiles and proudly announced "free, I free" pointing to himself.
For this reason and many others, we like having everyone in the same room while we are doing school. If you are schooling multiple grades at the same time, the older children are getting a review of concepts while the younger are learning them. The younger children are getting valuable exposure to new concepts while the older ones are learning. Plus, it just feels like Little House on the Prairie and that makes me smile.