I think it's an unrealistic proposition to expect an entirely sanitised, completely zero risk environment with dogs, or with any animals, or with any children, women or men to be fair.
Any child out there has the potential to strike and injure another child, yes to the point of hospitalisation if the scenario is particularly unfortunate or aggressive, but to take an instance of this and then use it as a rationale for why all children should stay on reins and at all times and no child should be allowed to run up to another or play together is clearly an absurdity.
There is clearly a balance to be had - if you are aware your child is likely to be aggressive of course you don't allow them to be in a situation where they could harm another until you have worked on the behaviours, together with experts as needed, and know how to manage the child yourself for future scenarios.
You don't issue an instruction that all children stay on reins all the time because you recognise the detriment to their exercise, activity levels, socialisation, stimulation and overall wellbeing, and you have a duty to your child in that respect. Dog owners of course have these same obligations to their dogs.
And you also recognise there can always be a first time - a horrifying moment that you didn't realise your child would be capable of. And you sincerely apologise to all affected and take the appropriate steps to try to ensure that doesn't happen again.
We live in a rich, complex world that we can't 100% sanitise, and that's partly what makes it rewarding and not sterile. There are horrifying and regrettable things happening every day and they won't always be preventable but need to be addressed if they occur.
I think the dog in question in this scenario should always be muzzled in public and I would tend to think if there is a second case it should be euthanised.