Pls could I check if we need to install Norton or similar on my daughters mac book and iMac desk top? She has been told she needs encrypted data etc.

I strongly second Cunnla's comments. I would add this. If you were to install Norton or another such app, you WILL significantly slow down the computer and it is likely that other serious problems will arise. This has been shown to be the case many times over. The Mac OS is very secure and has its own built in protection.

You do not need any antivirus third-party software on your Mac. Please read this. macOS - Security - Apple

And this: safe anti virus program - Apple Community

Encrypting the drive is a good idea to protect here data in case the computer is lost or stolen. macOS includes Filevault which can encrypt the whole drive. Launch System Preferences and click on "Security & Privacy". Then click on the "Filevault" tab. You will need to click the padlock on the lower left of the window to unlock the panel so you can enable Filevault. See this Apple article for details:

https://support.apple.com/en-us/HT204837

+3

Who told her that?