Can you please help me put block on childs phone so he can't download itunes?
Study this document and do everything it says: iOS: Understanding Restrictions
Also disable the Store in parental controls.
Also remove your credit card from the account, and just load a little at a time using gift cards.
Also make 100% clear to your kid that any purchases he/she makes will come directly out of his/her allowance / inheritance / hide.
All the restricitions I referred to are open by default. This forum frequently hears from people whose kids have run up big bills. Apple puts the onus on you to disable them.