I want to put £20 on my Son's Nexus as opposed to registering my credit card for him to just use any time.

It looks like you can only buy credit if you have a US account though as the options just do not appear on any UK Google Play web page like they do on the US site.

Has anyone worked out if you can add a set amount to your Google Play account?

Not being able to do this is a huge **** up by Google just like not having credit gift cards in shops to buy like you can get for iTunes. Parents do not want to leave their credit card on their children's devices FFS.

 

Havent tried , but what i do use is a pre-pay credit card . You top it up in the shops that have "paypoint" machines. You could just load it up with the £20 and let him use it like any other credit card. Not ideal as he could use it elsewhere , but at least its only 20 quids worth