It doesn't have to be constant. Please talk to your nurseries about ventilation, it is proven to make a difference to infection levels for airborne viruses (COVID, flu, chickenpox, measles, RSV and more). It won't eliminate transmission completely (kids get their faces close to each other, touch each other for non-airborne transmission) but it will make a difference.
Ideally they should have a CO2 monitor and keep levels below 800ppm. In France this is mandated for schools and nurseries, they have to take action to improve ventilation if levels exceed this. The UK is lagging it's our children who suffer the consequences.
Even better they should have HEPA filters running. These can be be DIY-ed for cheap, look up Corsi Rosenthal boxes.
Before you mention cost, nurseries will also benefit through less staff sickness, and kids benefit from better learning outcomes.
If you have school age kids, the same applies. Please encourage your school to sign up to the SAMHE project to monitor indoor air quality. www.samhe.org.uk
PS lockdowns were not bad for immunity. It is not encouraged to go around catching viruses, best to get your immunity through vaccination. Exposure to dirt however is a different matter.