A lot of half information being provided here so here's how to give your little cousin your DLC without giving your account detail
1) Create an account on your cousin's PS3, give it whatever name you want, 'PSN Share' for example.
2) Login to 'PSN Share' and linked the account to your PSN ID, but choose 'Do not save password' as the option.
3) Login to PSN and download the DLC, once it has been downloaded, exit PSN and install DLC.
4) Now you will have 2 account on the PS3, your cousin's account and 'PSN Share' the 'PSN Share' account is linked to your PSN ID but does not have any password so he can't log back in to your PSN account.
Your cousin can play the DLC on any account, but only if 'PSN Share' still exist, if it was ever deleted, you will not be able to access any contented downloaded from it.
Your cousin should not be able to get into your PSN, since there is no password, the local ID is not linked to your own PS3's local ID, its just exist seperately like another terminal.
Hope that helps.
