The value of investments, and the income from them, can fall as well as rise, and you may not get back the full amount you invest. Any tax efficiencies referred to are those applying under current legislation, which may change. 

For every child born on or after 1 September 2002 until 2nd January 2011, where the parent was eligible for Child Benefit, the Government issued a voucher worth at least £50 which could only be used to open a Child Trust Fund (CTF).  The money was invested in a tax-efficient fund in the child's name, and cannot be withdrawn until the child reaches age 18.