There wouldn't be a federal kidnapping charge available either, except Montgomery took the baby across a state line. The distinction between "kidnapping resulting in death" and plain old kidnapping is that the former is a death-penalty crime, even at the federal level, whereas the latter isn't. The feds are keeping their options open in case something goes wrong with the state murder charge in Missouri.
Indeed, the federal charge may be the simplest and quickest way to get this Montgomery person put down like the rabid b**** she is. There's no question the baby was kidnapped; there's no question the kidnapping resulted in a death; there's no question this Montgomery person planned the kidnapping, regardless of whether she wielded the knife. So the federal kidnapping charge is a) open-and-shut and b) sufficient to receive the death penalty.