I am ashamed to admit I haven't either and DS is 2.5.
I never actually had the voucher - had emergency c-section due to brain hemorrhagee (mine- all fine now) and had several days of morphine so was out of it- DP took Bounty pack (with voucher in I think) and just threw it all away. By the time I realised that was where the voucher would have been I just assumed without it I couldn't invest it which it terrible as money is tight and that should have been for him in the future.
Dreading explaining this one away when it comes to the time it should have matured.
The Bounty pack doesn't have the voucher in it, it has the leaflet/form to claim Child Benefit.
When you claim CB, it triggers the CTF which comes in a separate letter, it has your child's name on it and a unique reference number.
If you don't actively invest the CTF in your child's first year, the government does it for you in whatever their default plan is (possibly the Children's Mutual). I'm pretty sure you would be able to track down your child's CTF by calling the Inland Revenue with your Child Benefit reference number.
You get the voucher through the post once you've registered for child benefit - it's not in the bounty pack. If you haven't invested it by their first birthday, the government do it for you. I've got DS2's voucher in front of me as it arrived last week. Telephone number is 0845 302 1470 and website is www.childtrustfund.gov.uk Hope that helps!