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

chrome/browser/supervised_user/supervised_user_constants.h

Index: chrome/browser/supervised_user/supervised_user_constants.h
diff --git a/chrome/browser/supervised_user/supervised_user_constants.h b/chrome/browser/supervised_user/supervised_user_constants.h
index 1f06e89a0e77fc137d7b0e1b3c96179ca2771785..8571b99bc984c1d1791cae2430c33ec7dde6b56e 100644
--- a/chrome/browser/supervised_user/supervised_user_constants.h
+++ b/chrome/browser/supervised_user/supervised_user_constants.h
@@ -5,6 +5,10 @@
 #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
@@ -23,9 +27,6 @@ extern const char kUserName[];
 // (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.
@@ -33,6 +34,9 @@ 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_