Angular 2 unit testing - @ViewChild is undefined

I think it should work. Maybe you forgot to import some module in your configuration. Here is the complete code for test:

import { TestBed, ComponentFixture, async } from '@angular/core/testing';

import { Component, DebugElement } from '@angular/core';
import { FormsModule } from '@angular/forms';

import { ExampleComponent } from './test.component';
import { TimepickerModule, TimepickerComponent } from 'ng2-bootstrap/ng2-bootstrap';

describe('Example Test', () => {
  let exampleComponent: ExampleComponent;
  let fixture: ComponentFixture<ExampleComponent>;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [FormsModule, TimepickerModule.forRoot()],
      declarations: [
        ExampleComponent
      ]
    });
    fixture = TestBed.createComponent(ExampleComponent);
  });

  it('should recognize a timepicker', async(() => {
    fixture.detectChanges();
    const timepickerChild: TimepickerComponent = fixture.componentInstance.timepickerChild;
    console.log('timepickerChild', timepickerChild);
    expect(timepickerChild).toBeDefined();
  }));
});

Plunker Example

Make sure your child component does not have a *ngIf that is evaluating to false. If so, it will result in the child component as being undefined.