diff --git a/internal/mscs/msc2836/msc2836.go b/internal/mscs/msc2836/msc2836.go
index 13bb81c7d..e17fb1802 100644
--- a/internal/mscs/msc2836/msc2836.go
+++ b/internal/mscs/msc2836/msc2836.go
@@ -18,10 +18,13 @@ package msc2836
 import (
 	"bytes"
 	"context"
+	"crypto/sha256"
 	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
+	"sort"
+	"strings"
 	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
@@ -277,6 +280,8 @@ func (rc *reqCtx) process() (*gomatrixserverlib.MSC2836EventRelationshipsRespons
 	}
 	res.Events = make([]*gomatrixserverlib.Event, len(returnEvents))
 	for i, ev := range returnEvents {
+		// for each event, extract the children_count | hash and add it as unsigned data.
+		rc.addChildMetadata(ev)
 		res.Events[i] = ev.Unwrap()
 	}
 	res.Limited = remaining == 0 || walkLimited
@@ -358,8 +363,7 @@ func walkThread(
 	return result, limited
 }
 
-// MSC2836EventRelationships performs an /event_relationships request to a remote server, injecting the resulting events
-// into the roomserver as KindOutlier, with auth chains.
+// MSC2836EventRelationships performs an /event_relationships request to a remote server
 func (rc *reqCtx) MSC2836EventRelationships(eventID string, srv gomatrixserverlib.ServerName, ver gomatrixserverlib.RoomVersion) (*gomatrixserverlib.MSC2836EventRelationshipsResponse, error) {
 	res, err := rc.fsAPI.MSC2836EventRelationships(rc.ctx, srv, gomatrixserverlib.MSC2836EventRelationshipsRequest{
 		EventID:     eventID,
@@ -568,6 +572,8 @@ func (rc *reqCtx) getLocalEvent(eventID string) *gomatrixserverlib.HeaderedEvent
 	return queryEventsRes.Events[0]
 }
 
+// injectResponseToRoomserver injects the events
+// into the roomserver as KindOutlier, with auth chains.
 func (rc *reqCtx) injectResponseToRoomserver(res *gomatrixserverlib.MSC2836EventRelationshipsResponse) {
 	var stateEvents []*gomatrixserverlib.Event
 	var messageEvents []*gomatrixserverlib.Event
@@ -604,6 +610,38 @@ func (rc *reqCtx) injectResponseToRoomserver(res *gomatrixserverlib.MSC2836Event
 	}
 }
 
+func (rc *reqCtx) addChildMetadata(ev *gomatrixserverlib.HeaderedEvent) {
+	children, err := rc.db.ChildrenForParent(rc.ctx, ev.EventID(), constRelType, false)
+	if err != nil {
+		util.GetLogger(rc.ctx).WithError(err).Warn("Failed to get ChildrenForParent for adding child metadata")
+		return
+	}
+	if len(children) == 0 {
+		return
+	}
+	// sort it lexiographically
+	sort.Slice(children, func(i, j int) bool {
+		return children[i].EventID < children[j].EventID
+	})
+	// hash it
+	var eventIDs strings.Builder
+	for _, c := range children {
+		_, _ = eventIDs.WriteString(c.EventID)
+	}
+	hashValBytes := sha256.Sum256([]byte(eventIDs.String()))
+
+	err = ev.SetUnsignedField("children_hash", gomatrixserverlib.Base64Bytes(hashValBytes[:]))
+	if err != nil {
+		util.GetLogger(rc.ctx).WithError(err).Warn("Failed to set children_hash")
+	}
+	err = ev.SetUnsignedField("children", map[string]int{
+		constRelType: len(children),
+	})
+	if err != nil {
+		util.GetLogger(rc.ctx).WithError(err).Warn("Failed to set children count")
+	}
+}
+
 type walkInfo struct {
 	eventInfo
 	SiblingNumber int
diff --git a/internal/mscs/msc2836/msc2836_test.go b/internal/mscs/msc2836/msc2836_test.go
index d91f368ca..b56fd234d 100644
--- a/internal/mscs/msc2836/msc2836_test.go
+++ b/internal/mscs/msc2836/msc2836_test.go
@@ -4,10 +4,14 @@ import (
 	"bytes"
 	"context"
 	"crypto/ed25519"
+	"crypto/sha256"
+	"encoding/base64"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
 	"net/http"
+	"sort"
+	"strings"
 	"testing"
 	"time"
 
@@ -367,6 +371,21 @@ func TestMSC2836(t *testing.T) {
 		}))
 		assertContains(t, body, []string{eventF.EventID(), eventD.EventID(), eventB.EventID(), eventA.EventID()})
 	})
+	t.Run("includes children and children_hash in unsigned", func(t *testing.T) {
+		body := postRelationships(t, 200, "alice", newReq(t, map[string]interface{}{
+			"event_id":     eventB.EventID(),
+			"recent_first": false,
+			"depth_first":  false,
+			"limit":        3,
+		}))
+		// event B has C,D as children
+		// event C has no children
+		// event D has 3 children (not included in response)
+		assertContains(t, body, []string{eventB.EventID(), eventC.EventID(), eventD.EventID()})
+		assertUnsignedChildren(t, body.Events[0], "m.reference", 2, []string{eventC.EventID(), eventD.EventID()})
+		assertUnsignedChildren(t, body.Events[1], "", 0, nil)
+		assertUnsignedChildren(t, body.Events[2], "m.reference", 3, []string{eventE.EventID(), eventF.EventID(), eventG.EventID()})
+	})
 }
 
 // TODO: TestMSC2836TerminatesLoops (short and long)
@@ -457,6 +476,43 @@ func assertContains(t *testing.T, result *msc2836.EventRelationshipResponse, wan
 	}
 }
 
+func assertUnsignedChildren(t *testing.T, ev gomatrixserverlib.ClientEvent, relType string, wantCount int, childrenEventIDs []string) {
+	t.Helper()
+	unsigned := struct {
+		Children map[string]int `json:"children"`
+		Hash     string         `json:"children_hash"`
+	}{}
+	if err := json.Unmarshal(ev.Unsigned, &unsigned); err != nil {
+		if wantCount == 0 {
+			return // no children so possible there is no unsigned field at all
+		}
+		t.Fatalf("Failed to unmarshal unsigned field: %s", err)
+	}
+	// zero checks
+	if wantCount == 0 {
+		if len(unsigned.Children) != 0 || unsigned.Hash != "" {
+			t.Fatalf("want 0 children but got unsigned fields %+v", unsigned)
+		}
+		return
+	}
+	gotCount := unsigned.Children[relType]
+	if gotCount != wantCount {
+		t.Errorf("Got %d count, want %d count for rel_type %s", gotCount, wantCount, relType)
+	}
+	// work out the hash
+	sort.Strings(childrenEventIDs)
+	var b strings.Builder
+	for _, s := range childrenEventIDs {
+		b.WriteString(s)
+	}
+	t.Logf("hashing %s", b.String())
+	hashValBytes := sha256.Sum256([]byte(b.String()))
+	wantHash := base64.RawStdEncoding.EncodeToString(hashValBytes[:])
+	if wantHash != unsigned.Hash {
+		t.Errorf("Got unsigned hash %s want hash %s", unsigned.Hash, wantHash)
+	}
+}
+
 type testUserAPI struct {
 	accessTokens map[string]userapi.Device
 }