diff --git a/src/core/thread/mle.cpp b/src/core/thread/mle.cpp
index c8b629714..75ea6744a 100644
--- a/src/core/thread/mle.cpp
+++ b/src/core/thread/mle.cpp
@@ -374,7 +374,7 @@ ThreadError Mle::SetMeshLocalPrefix(const uint8_t *aMeshLocalPrefix)
 
 const uint8_t Mle::GetChildId(uint16_t aRloc16) const
 {
-    return aRloc16 & kChildIdMask;
+    return aRloc16 & kMaxChildId;
 }
 
 const uint8_t Mle::GetRouterId(uint16_t aRloc16) const
diff --git a/src/core/thread/mle_constants.hpp b/src/core/thread/mle_constants.hpp
index 2378b2396..c10ca376a 100644
--- a/src/core/thread/mle_constants.hpp
+++ b/src/core/thread/mle_constants.hpp
@@ -61,9 +61,10 @@ enum
 
 enum
 {
-    kChildIdMask                = 0x1ff,
-    kRouterIdOffset             = 10,
-    kRlocPrefixLength           = 14,   ///< Prefix length of RLOC in bytes
+    kMinChildId                 = 1,     ///< Minimum Child ID
+    kMaxChildId                 = 511,   ///< Maximum Child ID
+    kRouterIdOffset             = 10,    ///< Bit offset of Router ID in RLOC16
+    kRlocPrefixLength           = 14,    ///< Prefix length of RLOC in bytes
 };
 
 /**
diff --git a/src/core/thread/mle_router.cpp b/src/core/thread/mle_router.cpp
index 7112b53ff..dcdeed3a7 100644
--- a/src/core/thread/mle_router.cpp
+++ b/src/core/thread/mle_router.cpp
@@ -56,7 +56,7 @@ MleRouter::MleRouter(ThreadNetif &aThreadNetif):
     mAddressRelease(OPENTHREAD_URI_ADDRESS_RELEASE, &HandleAddressRelease, this),
     mCoapServer(aThreadNetif.GetCoapServer())
 {
-    mNextChildId = 1;
+    mNextChildId = kMaxChildId;
     mRouterIdSequence = 0;
     memset(mChildren, 0, sizeof(mChildren));
     memset(mRouters, 0, sizeof(mRouters));
@@ -1857,14 +1857,20 @@ ThreadError MleRouter::SendChildIdResponse(Child *aChild)
     SuccessOrExit(error = AppendSourceAddress(*message));
     SuccessOrExit(error = AppendLeaderData(*message));
 
-    aChild->mValid.mRloc16 = mMac.GetShortAddress() | mNextChildId;
-
-    mNextChildId++;
-
-    if (mNextChildId >= 512)
+    // pick next Child ID that is not being used
+    do
     {
-        mNextChildId = 1;
+        mNextChildId++;
+
+        if (mNextChildId > kMaxChildId)
+        {
+            mNextChildId = kMinChildId;
+        }
     }
+    while (FindChild(mNextChildId) != NULL);
+
+    // allocate Child ID
+    aChild->mValid.mRloc16 = mMac.GetShortAddress() | mNextChildId;
 
     SuccessOrExit(error = AppendAddress16(*message, aChild->mValid.mRloc16));
 
@@ -2287,7 +2293,7 @@ ThreadError MleRouter::GetChildInfoById(uint16_t aChildId, otChildInfo &aChildIn
     ThreadError error = kThreadError_None;
     Child *child;
 
-    if ((aChildId & ~kChildIdMask) != 0)
+    if ((aChildId & ~kMaxChildId) != 0)
     {
         aChildId = GetChildId(aChildId);
     }