You have exceptional children if they are on track with 20-30 minutes per child per day. My son could read, write, add, subtract (any numbers), and could sit and listen for 20 minutes at a time, when he was in kindergarten. He spent four hours in the morning learning social skills, and one hour each afternoon learning how to read/write and add/subtract. That was at least five hours a day.
His class was a public school kindergarten class, and every student in the class could do the same thing.
How much are your children missing? What are you doing with the rest of the school day? Seems to me that you are cheating them out of education.
Can they pass the same grade level tests that public school students? In Florida, they are required to pass these tests.