Hi WendyJane, All!
Many of us have the opposite problem, our animals tend toward being spoiled, and overweight!
Horses eat grass, and in the fotos you have posted, it looks as tho there is grass available. Let them have free access to all they want. One thing I have heard is that some of the high-altitude grasses are very tough and will cause rapid wear of the horses teeth. So the softer bladed varieties would be preferable.
Other than that, try corn. All of mine absolutely love fresh corn-on-the-cob; they eat it all, including the cob and husks. I would imagine they would eat the entire corn plant, given an opportunity. They also like fruits, and some melons. They like bread, pastries, and candy.
I would say that if you can eat it, your horses probably can as well. Just offer it to them and see if they like it. One thing you might do is take them for a walk in an uncultivated field where native plants are abundant, and see what they eat. They may nibble on just about anything, but it will be fairly obvious when they find something they like. Also try them on the leaves and stems of cultivated food plants, you may hit upon something they like that would otherwise be thrown out.
Some will tell you that starving horses will eat anything, but yours don't appear to be that hard up, and they will definitely tell you what they like and don't like. Mine are epicurean equines, and if the hay isn't up to _their_ standards, they will drag it out of the feeder and pee on it :-P
As far as working with them, absolutely do so. You can ride them as well, but gently, and not for extended periods, as they are still babies; young teens in human years. But they never forget, and if they know how to work under saddle now, they will know in a couple years as well. But be very particular about saddle fit. A poorly fitting saddle can cause all manner of problems, both physical, and emotional, and I would imagine particularly so in a young horse. If you can't find a saddle that fits properly, and can't justify the expense of buying one, only to replace it as the animal matures, I would suggest limiting your riding to very short, easy, bareback sessions. Then your butt will become a good gauge of how much discomfort is involved :-)
One on-line resource that has at least some information about horses in the less developed parts of the world is The Long Riders Guild. The Long Riders' Guild
You might look around their site, and see if there isn't a way to post questions specific to your area.
And I tend to preach, but . . . Relationship First. Pet 'em, groom 'em, love on 'em. Lots. Become their friends. The bond you build will last a lifetime, and they will do most anything for you.
Finally, I don't know about shipping into Ecuador, but surely the possibility exists. I'm totally sure the good people on The Horse Forum will be willing to help with things you are unable to find locally, so don't hesitate to ask.