No it is not obvious, that is why a visit, or a phone call to the PO is required to ask the question, but reading their blurb it does not have to be a PO account, it says they accept transactions for most major Banks, so it gives you more choice in finding an acceptable (I won't say good) interest rate.
Also these days I do not know at what age any financial institution will open solely in the child's name but that will not stop him depositing money himself. Only when it comes to a withdrawal he would need a parents signature.