Is there any way to have a second password to stop my children buying without permission. They need the original password to back up on icloud but I don't want them buying expensive apps. Can you help me with a solution?

You can'y have a second password on an iTunes account, but you can use Restrictions on an iOS device to hide the App Store app : Settings > General > Restrictions > Installing Apps 'off' (that will also stop you installing apps via a computer's iTunes, so you will need to turn that setting back 'on' to install apps and app updates from a computer)

And you can d hide the iTunes Store app via iTunes Store 'off' on that Restrictions screen (you can also prevent in-app purchases via Restrictions).