until the younger kids learned to play quietly when im working with one of them i would use my play pen and give them a certain toy that they could only play with at this time, or i'd use the taped box trick, after they learned what quiet time was we all just sit together and do the lesson,
We often do 1-on-1 time during nap....that's the easiest. But I have lots of little boxes of stuff for my rambunctious 4yo to do at the table while my 6yo does his phonics. We have really big fluffy pipe cleaners, a box of stuff to "magnet fish" (metal, plastic, wood, etc and you hang a magnet on a string and fish for the magnetic stuff in the box), beans and measuring spoons and cups, the little shaped magnets with pictures to make with them (I think the name brand ones are tanagrams?), duplos, aquadoodle (or just paper and water with a paint brush), some finger paint in a ziploc bag. Anything that will keep them occupied for 15 minutes or more.
Learning Centers and work/play boxes were very effective for my family. As were some Montessori activities. I believe it was scholastic that had a small book on learning centers - that I came across and used for this purpose. I would imagine there are free examples online tho. I also would have short lessons with frequent breaks - which is help in dividing your attention. I didn't have to deal with the issue for long tho.
How young? When my younger one(s) were still napping, that was when the bulk of 'school' was done - at least the workbook, sitting together at the table, type.
They gave up naps around age 3, by which time they could be entertained by a box of toys (some people have success with having toys that can ONLY come out during school time - keeps them interesting) or an educational DVD. Depending on how old your older child is, they may not need that much one on one time all at once. That sentence sounds weird. :)
I have them play on the computer, watch TV, or send them outside. We have a fenced in yard with a playground, trampoline and riding toys, and I can easily see them from the window. This is my favorite option!
If my youngest didn't interrupt me I would forget about him! He is a very content little guy and I can easily forget about him because I have three other boys doing grades 1, 2 and 4, who keep me very busy.
He plays with Legos and Playmobil. He loves to draw and scribble on paper and he has just started printing the alphabet and numbers too. He just turned 5.
Whatever works at any given time! Nap time, her playing at the neighbors house, assign one child to play with her while I work with the other then switch, watch for her to be entertaining herself and pull the other child for one on one at that time, let her watch TV, let her play on my iPad.
I think it really helps that I set up workboxes for the older kids. They are allowed to go through then in any order they want, so they work on all of their independent work, and when I see it is a good time for me to work with them I'll tell them, now's the time!