Ever felt like you were going 47 directions at once? Homeschooling multiple children in multiple grades can feel like this on many days. One way we have found to help focus things more and have a lot of fun is to approach a group of subjects from the information in one subject.
History is easy to use as your guiding subject. Whatever you are studying in history you could find literature to match the time period and all of a sudden you are talking about history in language arts. Science can be worked in as well, by looking at the geology of the area of the world you are studying or looking at what animals live in that place or particular time. Looking at exploring the technology of the time period is another way to link science with history.
Other subjects could be used as the guiding topic as well. Start with a particular book you want your children to read, add what was happening in the world at that time period or the place the book was set and history is added. Look at the technology, animals, weather, or geology that appear in the book and science is there too. We prefer to link other subjects with history, but it really could go any way you want it to. A good example of a literature based approach to including other subjects is The Prairie Primer by Margie Gray.
With multiple grade levels looking at similar books, or the same subject matter with more detail for older students works well. Your children can then talk and work together more than if they are studying completely different things. One aspect we have enjoyed is having the older students prepare presentations for their younger siblings.
Another great benefit we have found is the incidental learning that comes out of linking subjects. By reading a book linked with other subjects you may get started talking about the book and then a reference in the book may begin a whole new conversation on the history or science of the time. The same works in reverse, you may be talking history and be able to related it to the happenings in your book, beginning a whole new opportunity to learn.
The different angles to a particular subject provided by linking can give your children more opportunities and different opportunities to learn. Making part of a book meaningful for science or history purposes may help them really understand why a character did a particular thing in the story.
Have fun with combining subjects and see where it may take you.