Why can't we open up schools earlier since children do not seem affected by the virus?

Because the kids...: Kids want to their grandparents to live long enough to watch their high school graduations and then their college graduations. No kid wants to be the one who brings home the virus that makes Grandma and Grandpa disappear. It's a small planet. We are all interconnected. Good things other people do will help us, but the opposite is also true.