They require a prescription in the US, but I've heard that they are available without a prescription in Canada.

In Scouts anyway we expect Scouts with allergies to bring their own epipens, and let us know where they keep them. If stung, they use the epipen themselves: the only way for anyone else to apply the epipen is if they are unconscious. Its not a matter of training, although I suppose an inexperienced adult could screw up jabbing the Scout effectively; its a matter of liability too. And you're saving a life with an epipen, so follow with benadryl, get them off the trail and to medical assistance pronto.

I wouldn't bring an epipen as a just in case thing - in case a youngun experienced suddent unexpected anaphylaxis. A child is seizing on the ground, her throat is closing up - she isn't known to be allergic, do you really know what's going on, enough to justify the epipen injection? Without any history of allergic reactions, I'd do my best to interrupt the presumed histamine / antihistamine roller coaster with benadryl, and get medical assistance pronto.