Pay as you go mobile phones are great for kids and some operators such as Vodafone allow you to top-up online so that you can control the expenditure. Pay as you go is sometimes called Pay as you Talk (PAYT).
There are some great tariffs with unlimited texts etc. Vodafone gives FREE calls and texts all weekend if you spend £5 during the week.
You can also get the latest handsets from all the major networks. You can also buy a simple cheap pay as you go phone from Woolworths on the high st. The best bit though it no contract, no credit check and no commitment not even a monthly bill to worry about.
Checkout also the SIM FREE options.