Akka: How do I test parent sending msg to child?

ParentActorTest.scala

class ParentActorTest extends TestKit(ActorSystem("TestSystem")) with FunSuiteLike {
  test("Should delegate the important work to the client") {
    val underTest = TestActorRef(new ParentActor)

    underTest ! "Go do some work"

    // how do i test this?
  }
}