To briefly summarize: former slaves to a race of flying dragon-like humanoids (I've made some modifications to them based on feedback, but the gist is more or less the same) have escaped into the wilds to hide and gather strength. To stay hidden from the frequent airborne patrols, they make their homes in underground caverns and under covered areas of the world. They can and do venture above ground for food and supplies, but usually only do so at night or under cover of natural barriers like treetops. I assume since they're primarily underground that beasts of burden and crops are mostly out of the question, but feel free to correct me. It's probable that this group would eventually gain enough strength to be able to settle above ground, but that's only a future possibility.
Given their situation, how advanced could such a society hope to get?