You might start by gently, briefly touching them for just a second a little further back from the chin than where they are letting you now, just as they are taking the treat from your other hand so they start relating your touch to something good. It is important for them to begin to trust you so you can tend to their needs. Eventually you will want to be able to touch them everywhere, but that's going to take some time. I would touch and remove my hand as they take the treat and the next interaction later in the day or the next day move my touch back a little further until they are comfortable with that, progressing to the neck and withers as they allow. For now I would keep the touch brief.
We love our horses and so we desire to touch them! I saw this video (link below) a while back and I liked what this guy had to say about petting horses in general, so thought I'd share it.
Sounds like you are already very sensitive to your horses. They are very lucky to have ended up with you!!!