There are two types of JISA:
In a cash Junior ISA, you do not pay tax on interest. Although cash is guaranteed not to fall in value, annual inflation will have an adverse impact on its value.
In a stocks and shares Junior ISA, you pay no tax on capital growth or dividends. Investments are riskier but may mean growth is potentially greater than a cash product. However, unlike cash, the value of a stocks and shares Junior ISA can go up or down.