A back-of-fag packet approach is to use the Rule of 72 which holds that a rate of return divided into 72 gives the number of years your money takes to double.
If you use 7.2% as the annual growth, the investment doubles every ten years. This is actually below the long term average return from the stockmarket, but it's useful for illustration.
You put £2880 in now. The tax man adds another £720 in a month or two, making £3600 for the year.
In 10 years, that would be worth £7,200.
In 20 years, it would be worth £14,400.
In 30 years, it's will have doubled again to £28,800.
In 40 years, it's £57,600.
In 50 years, the point at which someone in English primary school Year 3 now would be able to access it under current rules, that one off £2,880 is worth £115,200.
For children, the one thing saving now really does have going for them is time and with the power of compounding (once described as the eighth wonder of the world) even small amounts now become enormous in 50 years.