diff --git a/stream-tests/src/test/scala/org/apache/pekko/stream/io/FileSourceSpec.scala b/stream-tests/src/test/scala/org/apache/pekko/stream/io/FileSourceSpec.scala
index 8f6a417568c..17da5ead664 100644
--- a/stream-tests/src/test/scala/org/apache/pekko/stream/io/FileSourceSpec.scala
+++ b/stream-tests/src/test/scala/org/apache/pekko/stream/io/FileSourceSpec.scala
@@ -24,6 +24,7 @@ import scala.concurrent.duration._
 import com.google.common.jimfs.{ Configuration, Jimfs }
 
 import org.apache.pekko
+import pekko.actor.ActorRef
 import pekko.stream._
 import pekko.stream.IOResult._
 import pekko.stream.impl.{ PhasedFusingActorMaterializer, StreamSupervisor }
@@ -81,6 +82,23 @@ class FileSourceSpec extends StreamSpec(UnboundedMailboxConfig) {
     f
   }
 
+  def fileSourceChildren(): Set[ActorRef] = {
+    SystemMaterializer(system).materializer
+      .asInstanceOf[PhasedFusingActorMaterializer]
+      .supervisor
+      .tell(StreamSupervisor.GetChildren, testActor)
+    expectMsgType[Children].children.filter(_.path.toString contains "fileSource").toSet
+  }
+
+  def expectNewFileSource(before: Set[ActorRef]): ActorRef =
+    awaitAssert {
+      val newChildren = fileSourceChildren().diff(before)
+      withClue(newChildren) {
+        newChildren.size shouldBe 1
+        newChildren.head
+      }
+    }
+
   "FileSource" must {
     "read contents from a file" in {
       val chunkSize = 512
@@ -258,30 +276,32 @@ class FileSourceSpec extends StreamSpec(UnboundedMailboxConfig) {
     }
 
     "use dedicated blocking-io-dispatcher by default" in {
+      val before = fileSourceChildren()
       val p = FileIO.fromPath(manyLines).runWith(TestSink())
+      val ref = expectNewFileSource(before)
 
-      SystemMaterializer(system).materializer
-        .asInstanceOf[PhasedFusingActorMaterializer]
-        .supervisor
-        .tell(StreamSupervisor.GetChildren, testActor)
-      val ref = expectMsgType[Children].children.find(_.path.toString contains "fileSource").get
       try assertDispatcher(ref, ActorAttributes.IODispatcher.dispatcher)
-      finally p.cancel()
+      finally {
+        watch(ref)
+        p.cancel()
+        expectTerminated(ref)
+      }
     }
 
     "allow overriding the dispatcher using Attributes" in {
+      val before = fileSourceChildren()
       val p = FileIO
         .fromPath(manyLines)
         .addAttributes(ActorAttributes.dispatcher("pekko.actor.default-dispatcher"))
         .runWith(TestSink())
+      val ref = expectNewFileSource(before)
 
-      SystemMaterializer(system).materializer
-        .asInstanceOf[PhasedFusingActorMaterializer]
-        .supervisor
-        .tell(StreamSupervisor.GetChildren, testActor)
-      val ref = expectMsgType[Children].children.find(_.path.toString contains "fileSource").get
       try assertDispatcher(ref, "pekko.actor.default-dispatcher")
-      finally p.cancel()
+      finally {
+        watch(ref)
+        p.cancel()
+        expectTerminated(ref)
+      }
     }
 
     "not signal onComplete more than once" in {