Not even the Children of the Forest know all the tunnels and they have lived for a thousands of thousands of human years (stated so in the books). If TEC knew the caves well, he would have shared the knowledge with the Children. Moreover, from what I remember, weirwood trees grant knowledge through what their carved faces (which aren't underground and are made by the Children, Free Folk or descendants of First Man) can see. If this is correct, and I remember nothing that suggests otherwise (correct me if I'm wrong), it's another point for the theory that TEC isn't all knowing. Gorne and Gendel lived a long time before our current events in Westeros, they lived in a time when Old Gods had more power and there were probably more Greenseers back then. And in the books, Ygritte stated that Gorne was the only one who knew the tunnels. Another point for the theory that TEC himself doesn't know the tunnels well.
If the Gendel's children aren't just another myth, perhaps there is a way to fight them. All the children of the dark fear fire and well, Gendel's children might be easier to fight than Wrights. Anyway, I wouldn't put my money on that one since I've only pointed it out for the sake of being thorough. They might just be a myth for all we know.