Not to be rude, but is that your oldest child’s child?
With the details you have posted about your younger three children’s academic accomplishments I have a hard time imagining them using government schools for their offspring.
Actually, I was flexible about kindergarten with my own kids. I saw it as a government sponsored play group( similar in many ways to the Department of Recreation summer programs for children.) If the child enjoyed it, I had no problem with it. My daughter has been homeschooling my grandson for several years now. He is reading fifth grade level books and is already well into second grade arithmetic even though he is only five. She is a wonderful mom and doing a great job homeschooling her kids.
I suppose my daughter is taking the same approach as I did. She's been doing the hard work of homeschooling at home, and sees no problem with kindergarten if the child likes it. One of my children didn't like kindergarten and only stuck it out long enough to participate in the Halloween school parade.
First grade, though, is an entirely different issue. I removed the oldest of my three homeschoolers after only a couple months (weeks?)( It was well before Christmas). Going to school chewed up far too much of his day. He came home tired. I felt obligated to continue our work at home which was several grades above what was being taught in school. The principal and teacher were completely unwilling to make any adjustments to accommodate his advanced level. We were doing both homeschooling and worthless school homework. Finally, I gave up and removed him from school. Homeschooling was looking pretty good to me, even though, as one of the earliest homeschoolers, I knew of no one who had or was homeschooling.