- They get tired from learning stuff. And the teachers know how to deal with them.
- They learn how to behave in a group. They start having actual friends.
- You get a bunch of friends who are in the same situation as you, and you can use their kids to cancel out yours. (AKA playdates)
- Having friends in the same boat will help your confidence a lot. They can help you directly with the kids, plus some of them will have older kids too, and they can tell you it gets better.
- As for your own time, try to be disciplined about the kids' bedtime. It helps a lot that school is tiring them out, and use that momentum to not let them decide when to go to bed. If you get them asleep before 8pm, how much different is your day really? You had to eat anyway, and you have to keep the house clean regardless. Your real problem is if they keep you up to 11pm each night, then your life is gone, and they end up in a cycle of having not enough sleep. Make them sleep, then code up your side project.
For example my kids started school at 4. I've put them in full time after school care as well, ending at 6pm. Now they're tired but awake when they come home and can eat a meal, then bath, then bedtime. You can read something to put them to sleep, then go and do your own stuff.
--Edit
Forgot to mention, the kid will learn how to read. From there you can give them a book and that will keep them quiet. Remember to teach them how to code as well, so they have proper tools to explore the world.