4-5 years ago it was good practice to maximise the ventilation in this way and encouraged on risk assessments.
We've had a parent with exactly this complaint. We're a group that does outdoor activities in public spaces as a regular part of our programme. Our risk assessments are within the national framework and passed within the organisation's hierachy. We suggested that if the parent was not happy with our risk assessment, then maybe we weren't the appropriate organisation for their child.
As leaders, our concern was that if we changed our practices to accommodate this parent's anxiety on this issue, how far would we have to go to appease other anxieties that may emerge in the future.
Some crimes are shocking because they are so very, very rare. Part of a risk assessment is considering the risk vs the benefit. This specific risk is very minimal in liklihood. The benefits of fresh air are good air quality, temperature regulation and health (against infectious illness). Logically if it's too dangerous to have the doors open, it's also dangerous for us to use the community facilities outside or activities like walks. We lock the other doors to our venue because they're not in direct view for constant supervision.
Perspective is important and excessive parental anxiety can be damaging to children's development. In our case, the parents in question didn't let their children do off-site activities which will long term be detrimental to their skills development and physical development.
They decided that they didn't like our risk assessment and withdrew the child.