Water down some neem oil and spray on your kale to get rid of the aphids. It's harmless to humans but smells truly vile (rotting onions) so you might have to wash your kale before you eat it if you haven't had much rain. Growing Borage closeby helps too as it attracts lots of aphid predators. I produces more nectar than any other plant. Careful if you have kids as it will swarm with bees. I usually have masses of bumble bees though and I think they are really cute.