Try working together for history. Many homeschool families have discovered that teaching the same topic to different age groups is more effective than trying to teach each age group a different topic. Each age group might have a different level of understanding and find different factors of each lesson to be interesting. For example, a first grader studying ancient history might find it fun to color cave drawings on a piece of stone while a high schooler might enjoy reading the biography of a famous archaeologist.
Read The Well Trained Mind. It's a guide for planning your classical homeschool curriculum to meet the needs of families with children in multiple age groups. Susan Wise Bauer is one of the authors and she also wrote the Story of the World History Curriculum designed for K-9th grade.
Try keeping a special box of toys, coloring books and other items that babies and toddlers can play with while you're working with homeschool curriculum with your older children.
Try working with the older kids while the younger children take their nap. Many homeschool families save math lessons, messy science experiments and art lessons for times when the littler children are sleeping.
Try a curriculum that's based in literature and story reading. The Well Trained Mind is one, but many parents prefer Sonlight. It's a boxed kit that has been a favorite of homeschooling families for many years for its simplicity and fantastic selection of books.