How do you keep the five year old inside during allergy season. You do not and you really do not want to deal with energy level of a kid who feels trapped inside.
So how do play outside with a kindergartener with severe allergies?
Again, the great benefit of homeschooling is setting your own schedule. To lessen chances of allergic reactions, we go to the park in the morning or late afternoon when pollen is higher in the sky, or on cloudy or cool days when it is not as prevalent. I also check http://www.pollen.com/ for daily accurate reports.
After we come back in from playing, we change clothing, saline his nose and bath. We also keep his hair cut low so we do not worry about trapped pollen from rolling (he is a boy after all) in the grass.