Martin Lewis warned that millions in the UK aged between 12 and 20 years could have more than £1,000 lying in a "dormant" savings account.
The Money Saving Expert highlighted the problem in his recent Martin Lewis Money Show episode and urged people to investigate.
He told his viewers that if they were born between September 1 2002 and January 2 2011, or have a child who was, they could have cash saved into a Child Trust Fund (CTF).
Child Trust Funds were tax-free savings accounts for children born in this time frame and were introduced to encourage parents to save for their child's future.
They are locked until the child turns 18 years old, and after this, they are able to access the money.
The UK Government provided an initial deposit of at least £250 to start every account, or £500 if your family were on a low income up until July 2010.
Plus a further £250 or £500 if the child turned seven before that date.
Between August 1 2010 and January 2 2011, the second payment was scrapped while the initial payment ranged from £50 to £500.
Either way, the Money Saving Expert said every account has a sum of money sitting in it.
He said: "So there are 6.3million of these and hundreds of thousands have lost track and on average a child trust fund is worth around £2,100.
"But I'm saying around £1000 because I think if you don't know you've got it and you've not added anything to it, you probably haven't managed it as well as it could be so it's probably got less in it."
How to find your Child Trust Fund
Martin went on to explain how to check if you have a Child Trust Fund, where it is and how to access it.
If you already know who your Child Trust Fund provider is, you can contact them directly. This might be a bank, building society or other savings provider.
If the trust fund is lost or you are unsure where it is, all you need to do is visit GOV.UK, search Child Trust Funds and complete an online form to find out where the Child Trust Fund is held.
Parents can do it for those under 18 years and you will need your Government Gateway and National Insurance number, however, if you're 16 years old you can do it yourself.
If a parent or guardian was not able to set up an account for their child, HMRC opened a savings account on the child’s behalf.
What to do once you have found it
Teenagers aged 16 or over can take control of their own Child Trust Fund if they wish, although the funds can only be withdrawn once they turn 18 years old.
Where children have a Child Trust Fund, families can still pay in up to £9,000 a year tax-free. The account matures once the child turns 18 years old and no further money can be deposited.
They can either withdraw the funds from the matured Child Trust Fund account or reinvest them into another savings account such as an ISA.
Martin's co-host Angellica Bell then shared the story of one viewer called Karen who was encouraged by the Money Saving Expert to go in search of her child's trust fund.
She said: "Karen says she forgot that this had been taken out and her 18-year-old recently got his and cashed it and got £1174.
"It was so easy to find and receive the money within a week."
