Took me a few hours to figure out because there's not much documentation in this (or maybe I just need more Gaia practice). At first I was trying to call defaultChild within index like this.defaultChild = whatever. Then I figured you probably had to do it using getPage and I was trying to pass it a route for defaultChild... Anyways, if anyone else is looking for the solution here it is:
(in constructor for index)
if(_userData.isLoggedIn) {
Gaia.api.getPage("index").defaultChild = "loggedIn";
}
Pretty easy... was just having trouble with the syntax of where I should put ".defaultChild ="