We should use UserID object to identify users instead of username.

chrome/browser/supervised_user/supervised_user_constants.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/supervised_user/supervised_user_constants.h"
+#include "components/user_manager/user_id.h"
+
+namespace {
+  const char kChildAccountSUID[] = "ChildAccountSUID";
+}
 
 namespace supervised_users {
 
 const char kCameraMicDisabled[] = "CameraMicDisabled";
 const char kContentPackDefaultFilteringBehavior[] =
     "ContentPackDefaultFilteringBehavior";
 const char kContentPackManualBehaviorHosts[] = "ContentPackManualBehaviorHosts";
 const char kContentPackManualBehaviorURLs[] = "ContentPackManualBehaviorURLs";
 const char kForceSafeSearch[] = "ForceSafeSearch";
 const char kGeolocationDisabled[] = "GeolocationDisabled";
 const char kRecordHistory[] = "RecordHistory";
 const char kSigninAllowed[] = "SigninAllowed";
 const char kUserName[] = "UserName";
 
 const char kSupervisedUserPseudoEmail[] = "managed_user@localhost";
 
-const char kChildAccountSUID[] = "ChildAccountSUID";
-
 const char kChromeAvatarIndex[] = "chrome-avatar-index";
 const char kChromeOSAvatarIndex[] = "chromeos-avatar-index";
 
 const char kChromeOSPasswordData[] = "chromeos-password-data";
 
+user_manager::UserID GetChildAccountSUID() {
+ return user_manager::UserID::FromUserEmail(kChildAccountSUID);
+}
+
 }  // namespace supervised_users