Tips to ensure you don't receive any unexpected bills:
1. Apple users - setup restrictions on your devices to disable in-app purchases (General > Restrictions > In-app purchases), ITunes on a Mac or PC can also be configured with restrictions.
2. Android users - ensure authentication is turned on - guide from Google on preventing unwanted purchases
3. Windows 10 users - make sure you take advantage of 'family settings' - guide from Microsoft
4. Keep your App store/Windows Store/Google Play password safe, and ensure your children can't change or guess it.
Ofcom have produced a series of video guides demonstrating how to prevent shocks from bills:
Link to Ofcom website