My kids are pre-trailer (late 20s), but I live down the street from a pediatrician who's an obsessed cyclist and fitness Nazi (good guy, but obsessed...). His kids are high school age now, and he's had them out on bikes, skis and snowshoes since they could walk. Until the developers effed up the neighborhood, we had access to miles of hiking trails just two or three minutes from his house. Many times, I've been hammering along on my mountain or road bike in temps in the 20s or even lower, all proud of myself, and he and his wife passed with the kids wedged into the trailers, bundled up to suit the weather. He says as long as their hands are warm and their noses and ears don't freeze, they'll be fine.