Well I finally got enough cash to my girls on the test server to purchase epic riding skill for both of them. I went back to the Netherwing quest starter in Shadowmoon Valley with Krystella and the quest was now open to her after she had the riding skill. The same is true for Kaliope. After purchasing the epic riding skill, the trainer in Moonglade was now offering the epic druid quest. So the moral of the story is... no epic riding skill, no quest.
From what I'm reading on the PTR forums and the little bit I experienced myself in starting the quests, the Netherdrake quest line is basically a faction grind. There are something like four repeatable quests that you can do to gain faction, with a limit on how many can be done each day. You'll need to be Exalted with Netherwing to get the mount, so this isn't going to be a quick process. If you're serious I suppose you could do every possible quest and reach Exalted in a few weeks. Either way, I'm definitely going to be wasting my time on this! Come on, what self-respecting rogue would ride something other than a Netherdrake?