Thanks for these replies. It's for my nephew who is 12.

Just a minimum amount which appears to be 30 UKP. For saving,  as he is already a keen saver and has said he would like some Bitcoin.

I would be nervous about him dabbling in dealing - are there safeguards to prevent this? 

I have also considered putting it in my own name and transferring it to him legally when he hits 18.   Is there any way of giving him "read-only" login rights to an account so he can check the value without being able to tinker with anything?