Q: Deleting letters in MAC OSX Name

If you mean the name that shows up in the Startup Disk section, then that's control by the name you give the boot volume. Click on the Desktop, select Finder from the menubar->Preferences->General->select to show HDs on the Desktop. Select the boot volume disk icon on the Desktop, hit the enter key, type in the new name.

Now, to preclude this kind of nonsense in the future, exert some parenting skills, ban the child from the machine for a week or so. Launch System Preferences->Accounts->unlock the lock->select Login Options->disable automatic login for everyone. Then, select your account and change the password to something he'd never be able to determine. Afterwards, create a standard account for him. Click the lock, click Show All->Parental Controls and set them appropriately for his use.