diff --git a/dev-docs/architecture.md b/dev-docs/architecture.md
index 8224137..42b138f 100644
--- a/dev-docs/architecture.md
+++ b/dev-docs/architecture.md
@@ -138,7 +138,7 @@ Each store is a separate LevelDB with JSON values. Keys are txids, addresses, or
 | `status` | `status` | `syncedBlockHeight`, `chainBlockHeight`, `startBlockHeight` |
 | `posts` | txid | Author address, text, timestamp |
 | `postParents` | child txid | Parent txid (reply) |
-| `postChildren` | parent txid | Reverse index for replies |
+| `postChildren` | `parentTxid:childTxid` | One reply link per parent–child pair |
 | `likes` | like txid | Post txid, liker, optional tip |
 | `names` | address | Display name + provenance txid |
 | `profiles` | address | Profile text |
diff --git a/dev-docs/psf-memo-db.md b/dev-docs/psf-memo-db.md
index 8d9e44e..0727586 100644
--- a/dev-docs/psf-memo-db.md
+++ b/dev-docs/psf-memo-db.md
@@ -71,7 +71,7 @@ All routes are under `/level` with a consistent CRUD pattern generated from `ENT
 |-------|---------|-----------|-------------|
 | `post` | `txid` | `postData` | transaction id |
 | `postparent` | `txid` | `parentData` | child txid |
-| `postchild` | `txid` | `childData` | parent txid |
+| `postchild` | `key` | `childData` | `parentTxid:childTxid` |
 | `like` | `txid` | `likeData` | like txid |
 | `name` | `addr` | `nameData` | cash address |
 | `profile` | `addr` | `profileData` | cash address |
@@ -187,7 +187,7 @@ Common fields on indexed documents:
 | profilePic | addr | `url`, `txid`, `seen`, `addr`, `blockHeight` |
 | like | txid | `addr`, `postTxid`, `seen`, `tip`, `blockHeight` |
 | follow | composite key | `followerAddr`, `followeePkHash`, `unfollow`, `txid`, `seen`, `blockHeight` |
-| postParent / postChild | txid | `parentTxid`, `childTxid`, `blockHeight` |
+| postParent / postChild | txid / `parentTxid:childTxid` | `parentTxid`, `childTxid`, `blockHeight` |
 | room | composite key | `room`, `txid`, `seen`, `type`, `blockHeight` (+ `addr` for follows) |
 | processError | txid | `error`, `ts`, `blockHeight` |
 
diff --git a/src/adapters/adapters-index.js b/src/adapters/adapters-index.js
index 7d6f19b..dde951e 100644
--- a/src/adapters/adapters-index.js
+++ b/src/adapters/adapters-index.js
@@ -21,7 +21,7 @@ class Adapters {
 
     this.postDb = createEntityDb('post', 'txid', 'postData')
     this.postParentDb = createEntityDb('postparent', 'txid', 'parentData')
-    this.postChildDb = createEntityDb('postchild', 'txid', 'childData')
+    this.postChildDb = createEntityDb('postchild', 'key', 'childData')
     this.likeDb = createEntityDb('like', 'txid', 'likeData')
     this.nameDb = createEntityDb('name', 'addr', 'nameData')
     this.profileDb = createEntityDb('profile', 'addr', 'profileData')
diff --git a/src/use-cases/action-types/helpers.js b/src/use-cases/action-types/helpers.js
index 0ef0f04..6a65987 100644
--- a/src/use-cases/action-types/helpers.js
+++ b/src/use-cases/action-types/helpers.js
@@ -57,3 +57,7 @@ export function followKey (followerAddr, followeeAddr) {
 export function roomKey (roomName, txid) {
   return `${roomName}:${txid}`
 }
+
+export function postChildKey (parentTxid, childTxid) {
+  return `${parentTxid}:${childTxid}`
+}
diff --git a/src/use-cases/action-types/reply.js b/src/use-cases/action-types/reply.js
index 94cb2e6..bcb9920 100644
--- a/src/use-cases/action-types/reply.js
+++ b/src/use-cases/action-types/reply.js
@@ -1,4 +1,4 @@
-import { utf8FromPush, txHashFromPush, logProcessError } from './helpers.js'
+import { utf8FromPush, txHashFromPush, logProcessError, postChildKey } from './helpers.js'
 import { MAX_REPLY_SIZE } from '../../lib/memo-codes.js'
 import { handlePost } from './post.js'
 
@@ -24,7 +24,7 @@ export async function handleReply (ctx) {
   }
 
   await adapters.postParentDb.create(txid, { parentTxid, childTxid: txid, blockHeight })
-  await adapters.postChildDb.create(parentTxid, { parentTxid, childTxid: txid, blockHeight })
+  await adapters.postChildDb.create(postChildKey(parentTxid, txid), { parentTxid, childTxid: txid, blockHeight })
 
   await handlePost({ ...ctx, decoded: { ...decoded, pushDatas: [pushDatas[0], pushDatas[2]] } })
 }