I ve discovered that by focusing on one area and incorporating fun into their studies, my children seem to retain and understand more than they did before. I believe that it allows them to fully engross themselves into a topic and sometimes even become the topic in playacting, therefore they connect better and remember more. Some people try to incorporate math into unit studies. We do a combination of literature, science and social studies units, even though you can incorporate science into a social studies unit and vice versa I still think you need the focus on each area to truly get a balanced education. To do unit studies with more than one age group, just modify the activities somewhat.
Younger children can read easier books, make simpler art projects, etc. than older children.
I often have my oldest show and explain things to my youngest which helps her to be a patient and thoughtful person and helps the younger child to learn. When teaching younger children, the older ones can be reading or working on projects on their own. It helps me to keep things straight when everyone is basically studying the same topic. I keep the expense for a unit study to a minimum by using the library instead of buying unit studies or books, creatively looking for supplies from what we already own or thrift stores, and looking for discounts and free days for field trips. For each big unit study, I usually budget for one bigger craft or field trip, but it isn t necessary.