Short term - Open windows more often
Long term - Replace windows with modern units with trickle vents.
As a rule I leave at least one window at the front, and one window at the back of each floor of my properties cracked, as well as open trickle vents. New double glazing can be locked when on a crack so you're still fully insured.
I have countless issues with my tenants about condensation. I've been in some where moisture is literally running down the walls because all the windows are closed, there's wet washing all over the house and the kettle's on every 5 minutes. Not to mention the pints of moisture we breathe out all day.
The better sealed a house is (modern houses are very well sealed) the worse this problem becomes. Modern glazing can help, but really we need to be teaching people that as much as it seems counter intuitive, no matter what the weather, or quality of glazing, your home needs airflow.
Not to mention the health implications for kids when mould inevitably occurs.
Why we'd rather teach kids algebra than a few basic ways to look after yourself and your home ill never know, but there it is.