It cannot be accessed by them until they reach 18, at which point it becomes an adult ISA. The annual allowance in the 2015/16 tax year is £4,080, and can be split into cash and/or stocks and shares JISAs.
It is tax-exempt, and cannot be accessed until 10 years before the state retirement age. The annual maximum contribution allowance is currently £2,880, and the Government automatically adds up to 20% tax relief (maximum £720).