It's becoming more and more common for parents to buy smartphones for their children to help keep track of them and for use in emergencies.

However, the trend also comes at a cost, with kids racking up huge calling and texting bills, playing games and watching videos during school time (and after bed-time), and being exposed to text- and cyber-bullying.

Help is available, though, in the form of a free app from Vodafone called Vodafone Guardian.

Released in eight countries last December, including New Zealand, Vodafone Guardian allows parents to set the times of day in which their child can and can't use their phone for calling, receiving texts and accessing the web browser.

The app can also bar calls from numbers that aren't in their contacts list, decide the time of day apps such as games and video players can and can't be used, and block texts from individual contact numbers to help address bullying.

There are three pre-programmed age settings for children, 7-8, 9-10 and 11-12. All settings can still be tailored within these, and in emergencies, the restrictions can be overridden for 15 minutes while the child makes a call.

We'll be featuring Vodafone's Guardian app as our App of the Week in the next few weeks. In the meantime, have you tried it on your child's phone? Was it useful? Should children that young even have smartphones?