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

chrome/browser/supervised_user/supervised_user_constants.h

 // 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.
 
 #ifndef CHROME_BROWSER_SUPERVISED_USER_SUPERVISED_USER_CONSTANTS_H_
 #define CHROME_BROWSER_SUPERVISED_USER_SUPERVISED_USER_CONSTANTS_H_
 
+namespace user_manager {
+class UserID;
+}
+
 namespace supervised_users {
 
 // Keys for supervised user settings. These are configured remotely and mapped
 // to preferences by the SupervisedUserPrefStore.
 extern const char kCameraMicDisabled[];
 extern const char kContentPackDefaultFilteringBehavior[];
 extern const char kContentPackManualBehaviorHosts[];
 extern const char kContentPackManualBehaviorURLs[];
 extern const char kForceSafeSearch[];
 extern const char kGeolocationDisabled[];
 extern const char kRecordHistory[];
 extern const char kSigninAllowed[];
 extern const char kUserName[];
 
 // A pseudo-email address for systems that expect well-formed email addresses
 // (like Sync), even though we're not signed in.
 extern const char kSupervisedUserPseudoEmail[];
 
-// A special supervised user ID used for child accounts.
-extern const char kChildAccountSUID[];
-
 // Keys for supervised user shared settings. These can be configured remotely or
 // locally, and are mapped to preferences by the
 // SupervisedUserPrefMappingService.
 extern const char kChromeAvatarIndex[];
 extern const char kChromeOSAvatarIndex[];
 extern const char kChromeOSPasswordData[];
 
+// A special supervised user ID used for child accounts.
+user_manager::UserID GetChildAccountSUID();
+
 }  // namespace supervised_users
 
 #endif  // CHROME_BROWSER_SUPERVISED_USER_SUPERVISED_USER_CONSTANTS_H_