[fred69]

Further to the Hygiene Hypothesis based approach, add exposure to cattle. They are another common factor during evolution and have a microbial footprint unlike common pets. Over years there have been a number of studies on this, sorry to not have a list of URLs. Take your kids -- and yourselves -- to visit farms and petting zoos.

And my anecdotal input: grew up on a livestock farm, also spending a lot of time in woods and river. No allergy problems at all. Moved to a city and had annoyance-level problems ever since. Decades of observation lead me to suspect particulates (engine soot, tire dust, etc.) as significant antagonists beyond the more commonly cited biological villains. Any chemical with a big-enough stretch of molecule to match what an immune cell is using as its search key.