When I was living in Brazil fresh air was part of my daily life. I can probably count on my fingers the amount of days that we did not open any windows at my house. The concept of living inside closed quarters without fresh air, did not exist. And because most people did not have air conditioners when I was growing up (and many still don't to this day) the windows were always open, specially in the early morning, to let the cool air come in before the sun got too hot. Even in the winter, we would always open the windows a little bit to renew the air, specially if anyone was sick with the flu. And then I moved to America.
When I first moved to New Jersey, after getting married, it was December, winter and very cold. With the heating system running strong to keep our apartment warm, the windows stayed down. And down they stayed all winter! That means about 3 months! Then when summer came I noticed that many people in our apartment complex still had their windows down because they were running their air conditioner. More 2 months with the windows down! And lets not forget that some people are allergic to pollen and have to keep their windows down during Spring and sometimes during Fall too. That pretty much covers the entire year!
Research shows that indoor air pollution is worst than outdoor air pollution, because of all the off gassing produced by cooking smokes, furniture, mold, dust mites, among others. So that is not encouraging! Some plants have the ability to clean the indoor air, and they make great decoration items too. Something encouraging!
In our apartment now we have a pump heating system that I believe pumps air from the outside to heat the place, so that could be a bonus. But when it comes to kids, I have my own research results. Kids are happier outside!! They thrive on fresh air, on open spaces, on Nature. It has this calming effect. When I feel like they are very close to start climbing the walls inside the apartment, all that it takes is a walk to the nearest playground, some running, biking and breathing and they, like computers, restart fresh anew!!