Well, the aphids are still around, but the most effective means I've found of eradicating them is the old-fashioned rushing water method. I simply pour water along the stem of the plant and do a little squishing as I go. It is still rather disgusting, but at least I've been able to save the Swiss Chard. And the kale seedling is also suriving.
The "deadly tea" I brewed seemed to have no effect on them, but I will say that I still found it a very satisfying procedure.