that risk is much less from the vaccine than from covid

Do you have evidence for that? In children, if infected, the virus is apparently not replicating in any significant way, therefore children hardly develop symptoms. In contrary, after the vaccination (the dose is not even reduced for children!) a massive amount of the spike proteins is produced and floating all over the body, not only in the respiration tract where the virus normally is but in the blood vessels where the dangerous throboses can happen. So you really can´t compare those 2 events and generally say, which event cause more exposure to that poisonous proteins. One (infection) is a "normal" one which our immune system is built for, the other one a treatment not yet fully licenced, about its effect we still don´t know much. And you still want to let children take part in such an experiment?
It is a difference if you treat children with e.g. cancer with an experimental drug, there a risk might be acceptable. But not healthy kids with almost no risk.