Never worked on it myself. So I can't say.Hopefully someone can be of more direct guidance.
My advice as to how I would go about it would be to do some studying on epliepsy itself. Learn what causes it. What exactly in the body is affected by it. Where it stems from. Any details you can find.
Then meditate on and/or logically deduce where you feel the best areas to treat would be.Also (with permission if you believe in it) try to find a way to connect with the youngin and see if you can find the actual spiritual root of the problem.
I feel this is gonna be a very long term treatment thing. Learning as you go. Even someone with experience can only tell you so much because individuals react to Reiki quite differently.