What security do I need to set up for the dcs on computers/ipads?

[MrsFogi]

Somewhere along the line I seem to have been left behind by technology so I'm feeling a bit lost as to what I need to do. I want to give my dds more access to computers but want to ensure they don't stumble on unsuitable material etc. We are entirely on Macs (iPad, iMac and MacBook). What would you set up if you were me before letting them loose (they are 8 and 10)?
In particular are there settings I should be switching on or off, apps I should use or services I should sign up to (e.g. net nanny)?

At least on the iPad you can turn of the app store, YouTube and internet browser. This would restrict them to only using apps you approve and buy for them.