Keeping younger ones busy while homeschooling older children is not easy. But there are activities out there that are cheap and lots of fun.
Here our resident four year old is practicing his numbers in shaving cream. It's a little easier than holding a pencil for thos tricky numbers at this age and it's fun!
He does have a workbook where he is learning his letter sounds but activities like this keep learning fun and little hands busy.
When he was done with his numbers we let him get a couple of his cars to play with in the shaving cream. He was making roads and pretending to give baths to the toys.
He also spends time "reading" books while the others do their seatwork. I belive it is good to give preshoolers times of structure. It teaches them consideration for others and builds their ability to sit still for periods of time. If you know this boy, being able to sit still (if only for a time) is a good thing.