add ChildAggregation test

Signed-off-by: Siva <ychnagasivareddy@gmail.com>

java-client/src/test/java/org/opensearch/client/opensearch/integTest/RequestTest.java

@@ -374,6 +374,28 @@ public void testSubAggregation() throws IOException {
 
     }
 
+    @Test
+    public void testChildAggregation() throws IOException {
+
+        String index = "test_child";
+        String question = "question";
+        String answer = "answer";
+        Map<String, List<String>> relations = Collections.singletonMap(question, Collections.singletonList(answer));
+
+        highLevelClient().indices().create(c -> c.index(index).mappings(m -> m.properties("join", p -> p.join(j -> j.relations(relations)))));
+
+        highLevelClient().index(i -> i.index(index).id("1").document(new Question("exists")).refresh(Refresh.True));
+
+        highLevelClient().index(i -> i.index(index).id("2").routing("1").document(new Answer("true", "1")).refresh(Refresh.True));
+        highLevelClient().index(i -> i.index(index).id("3").routing("1").document(new Answer("false", "1")).refresh(Refresh.True));
+
+        SearchRequest searchRequest = SearchRequest.of(r -> r.index(index).size(0).aggregations(answer, a -> a.children(c -> c.type(answer))));
+
+        SearchResponse<Void> searchResponse = highLevelClient().search(searchRequest, Void.class);
+
+        assertEquals(2, searchResponse.aggregations().get(answer).children().docCount());
+    }
+
     @Test
     public void testGetMapping() throws Exception {
         // See also VariantsTest.testNestedTaggedUnionWithDefaultTag()
@@ -469,4 +491,48 @@ public void setPrice(double price) {
             this.price = price;
         }
     }
+
+    public static class Join {
+        public String name;
+        public String parent;
+
+        Join() {}
+
+        Join(String name) {
+            this.name = name;
+        }
+
+        Join(String name, String parent) {
+            this.name = name;
+            this.parent = parent;
+        }
+    }
+
+    public static class Question {
+        public String title;
+        public Join join;
+
+        Question() {}
+
+        Question(String title) {
+            this.title = title;
+            this.join = new Join("question");
+        }
+    }
+
+    public static class Answer {
+        public String body;
+        public Join join;
+
+        Answer() {}
+
+        Answer(String body) {
+            this.body = body;
+        }
+
+        Answer(String body, String parent) {
+            this.body = body;
+            this.join = new Join("answer", parent);
+        }
+    }
 }