A Midland has a black "ladder" pattern going down the (signature Brown Snake) dorsal stripe. A Northern lacks the "ladder" and instead has a clean, tan-ish dorsal stripe.
Interestingly, both sub-species can inter-breed without a problem and actually do so frequently in the wild, producing offspring of a 3rd species with traits from both parents (probably baby snakes with both "ladder" patterns and specialized teeth).
P.S. I would love to see pics so I can identify it and also to see if it's as nice looking as that kid's one.