How big (and numerous) are the fairies? They would need to be able to drop so much flechette (<-correct spelling) of at least enough size to puncture through armor and skull from very high up after carrying them however far (endurance issues) that it would largely be likely not particularly effective and at best maybe moral dropping if they manage to do some damage, but without modern production means, that also means the flechette has to be produced one at a time (notably by the fairy's or their allies blacksmiths, and that's a lot of metal that could be going into much more productive areas).
It would be better to have the faeries act as skirmish units and pull off hit and run tactics as well as saboteurs. Their small size, unless they glow, make them much harder to spot, and give them an oversized razer blade (shad covered this for them even) and they can sneak up, cut throats and disappear before being noticed. All it would take for a saboteur would be a little poison in the enemy's water supplies, fires started in their dry goods (such as fodder for their animals if they have fodder, and if not their dry foods then) or a few commandos who go into the enemy camp and kill a bunch of soldiers in their sleep. The faeries even have multiple ways of sneaking in (overhead or from the sides.
Actually going back, rather than dropping flechette on them, waiting until the enemy has set up camp, then dropping fire pots on them (it would probably take more coordnation, like teams of fairies working together to carry them and light them before dropping them) would be way more damaging for moral (also making the flechette attack during the night would be better, especially if they simply hit obvious command tents).