Best solution: bring them inside. I know you have ruled this out and you may have very good reasons, but I'll just encourage you to double check. Maybe just the bikes inside and not the trailer? It's not actually even necessary between the other kid-transporting options you have, but I digress. I know a lot of times "there's not enough room" but there can be creative solutions to that. Our house and plenty of our friends' houses have a bunch of bikes there when you come in and that's just how it is.
Leaving them exposed to the elements will definitely wear them out fast, especially in the winter (at least if you have snow).
Any coverage is better than nothing. I'm a little confused about what space you have to work with, but the best thing for protection would be to build or buy a shed. The pop-up canopy tent idea could be workable (would it have walls though? Walls would make a big difference), though the material may wear out with constant use, and if the sides don't go all the way to the ground, I find just the humidity and/or rain splash can cause damage over time even if they are mostly covered.
They're kind of a pain to put on, but those individual covers like people put on motorcycles would go a long way toward protecting them.
If you are determined not to bring them inside, the least you can do is frequently clean and lubricate the drivetrain, and make sure you attend to the paint job and/or any small rust spots.
Good luck!