Child self assessment

[nickjc]

My child has a bare trust set up many years ago in their name, with myself (parent) as trustee. The funds originate from monthly contriubutions from grandparents, and invested in index funds, intermittently sold for things such as school fees. This year, as the CGT allowance has dropped down to £3000, this has triggered a CGT bill. HMRC does not seem to be set up for this, and registering for a self assessment on line is not possible (you have to be over 15).
Can anyone run through the approved method to get a UTR / NI number to proceed with the self assessment process.