This, as my anecdotal evidence, and every medical text about it, contradicts your ideas. Please consult a doctor before doing anything you suggested to your children or yourself, because your suggestions are /dangerous/.
(I'm not affiliated with https://www.wyndly.com/, but that could be a good place to start for those unfamiliar.)
Also, suggestions like those usually bring out the quackery in people, with things like "Oh, hayfever? Let's cure it by doing summer holidays on the farm!" and "Oh, peanut allergy? I have a sandwich for you, kid!".
Yes, allergies always develop after exposure. But not all exposure is created equal! Some types of exposure tend to cause allergies while other types tend to protect against allergies. Immunotherapy is the right type of exposure, under the guidance of an allergist.
> I suspect that in twenty years it will be standard to start infants on sublingual immunotherapy for all the common allergens (literally putting a tiny, strictly controlled amount of the allergens under the tongue once a day, simple as that) as a prophylactic
Also, even therapeuthic or diagnostic exposure can cause allergies, there is no safe exposure. Just safer and less safe.