Safe vs dangerous in these situations are always limited to one very specific thing, and completely ignores the rest, both comparative risks (such as driving to the airport being riskier than flying in a plane) and also absolute risks that can be mitigated (never going outside removes almost all the risk of catching a disease)
It's nothing about all risk - we have no idea on the risks of isolation in a pandemic - we have loads of evidence that isolation is hugely correlated with a higher chance of death, but don't know for sure that applies to pandemic isolation, or really what the long term effects of isolation in children is particularly. The only kids who are normally isolated have quite a lot of other things that harm them (bullying, abuse, disability etc. which is why they're isolated)
So because no-one is able to quantify those risks of not doing something they purely focus on the thing they can control - not going to school, removes the risk of catching a disease. Regardless of what that risk is, they can control it, and the alternative is so unknown, and possibly unrecognisable to them at all, that it's not considered.
As someone who manages a mental illness which is being made more and more debilitating by the isolation caused by lockdown, I'm sure I'm judging the risk that if my child suffers the same as much more important a consideration, 'cos it's right there screaming in my head.
I don't actually care if the schools go back, it's kid isolation that I want addressed, but schools going back appear to be the only route to that. Even now we can't meet another kid in the park (since the advice is meet a single person from another family, so no child who needs a parent to get them there.)
People who worry on educational aspects and struggle with that at home will want the school back to meet those.
None of this is wrong, it's just psychology, people can't know everything, they can't control everything but end up focusing on things they can control - or fears that they can't control, but can address. So for many, the fear of the disease (even when it's by proxy and it's not their own child they're worried about) override the other risks that just aren't ones that they grasp.
It's why we need more information provided, not just the risk of a child catching a disease, but what the risks are about them not being in school, people only seem to think about lost education.