To be honest it mostly comes down to planning (hence our set school times) and making sure that each child gets some one on one time for the topic they are struggling with. For example - I might be working one on one with Rose on her letter and phonetic reading while the older two (Lilly and Sunflower) are researching and writing up some history work and Tulip is doing copy work. Or while Lilly is working on a new math topic, Sunflower and Tulip will be working on their geography maps while Rose does letter tracing.
I also rely on the girls teaching each other - this is a good way for the older girls to prove they understand a topic by sharing their knowledge with one of their sisters (and a nice way of freeing me up for some one on one time where needed)
The other lifesaver for me is joint lessons - where possible I use joint lessons so all the girls will sit together to discuss, read and narrate then be sent off to do some individual work/research based on what we've just learnt - science, history, geography and even math all lend themselves to this format in our house.
Science is a easy example - when I did the life cycle of stars presentation we all sat together,
did the activity together, discussed it together, then they split off to do some individual work on it which consisted of the older girls writing (in their own words) in their science journals and the younger ones working with the 3 part cards, stamps and star crafts.
History usually involves reading about it together, individual narration (they either tell me in their own words what we've just read or answer questions on what we studied) and then, depending on age, write it up or do a craft/draw a picture.
Geography also works well as a joint lesson - we study the country together then they split with the older girls doing some more research about life in that country while the younger ones trace a map of the country or draw the flag etc.
Math is a great example - when we studied square numbers together the oldest 3 each drew diagrams or suck square stickers to represent them while Rose went off to the kitchen and used Shreddies (a square shaped breakfast cereal) to match the square numbers.
The joy of joint lessons for me is the way they all grasp the knowledge better when working together - Rose was 3 1/2 when we did the life cycle of a star lesson but she wanted to know more about it than her bigger sisters did and we ended up reading star and space books to her for weeks.