Sub-Module child routes not working ("Error: Cannot match any routes.")

I'm using Angular 6.0.3. I have a sub-module called "Admin" with three components. Admin's root component ("AdminComponent") route works fine (path: ""), but I can't seem to trigger any of the others. Why can't Angular find my routes?

routes.ts

import { Routes } from "@angular/router";
import { SplashComponent } from "./splash/splash.component";
import { LoginComponent } from "./login/login.component";
import { WatchComponent } from "./watch/watch.component";

import { AdminComponent } from "./admin/admin/admin.component";
import { HomeComponent as AdminHomeComponent } from "./admin/home/home.component";
import { AddSongComponent } from "./admin/add-song/add-song.component";
import { AdminGuard } from "./auth/admin.guard";

export const appRoutes: Routes = [
  {
    path: "",
    children: [
      { path: "", component: SplashComponent, pathMatch: "full" },
      { path: "login", component: LoginComponent, pathMatch: "full" },
      { path: "watch", component: WatchComponent, pathMatch: "full" },
      {
        path: "admin",
        component: AdminComponent,
        pathMatch: "full",
        canActivate: [AdminGuard],
        children: [
          {
            path: "",
            component: AdminHomeComponent,
            pathMatch: "full",
            outlet: "admin"
          },
          {
            path: "add-song",
            component: AddSongComponent,
            pathMatch: "full",
            outlet: "admin"
          }
        ]
      }
    ]
  }
];

admin/admin/admin.component.html

<router-outlet name='admin'></router-outlet>

admin/home/home.component.html

<div>

   <a mat-raised-button [routerLink]="['add-song']">Add new song</a>

</div>

note: I've also tried [routerLink]="[{ outlets: { admin: ['add-song'] } }], still route not found.

from Sub-Module child routes not working ("Error: Cannot match any routes.")