Let’s assume that you have a structure like following:

- your.component

  - your-child1.component

  - your.child2.component

-your.pipe

-your.module

In your.module you will import and declare all your three components and the pipe:

import { NgModule }      from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { YourComponent }  from './path.to.component';
import { YourChild1Component } from './path.to.component';
import { YourChild2Component } from './path.to.component';
import { YourPipe } from './path.to.component';

@NgModule({
  imports:      [ BrowserModule ],
  declarations: [ YourComponent, YourChild1Component, YourChild2Component, YourPipe ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

Same way you should do in your test class for your tests to work, you import the components and the pipe and add them to your

TestBed.configureTestModule(declarations: [Your component, YourChild1Component, YourChild2Component, YourPipe])