It's been noted that kids for whatever reason are not as susceptible but it's not just kids in the schools.
If schools were only filled with children, and those children didn't go home to their parents, we could open the schools yesterday. The problem is that children still carry the virus, and are capable of, in a more limited fashion, spreading it to others.
That's why Bidens plan includes money to upgrade ventilation systems, provide for social distancing, disinfecting and upgrading schools. And of course how that there is a vaccine, to prioritize inoculating teachers.