my children have put a pin code on their user mii's but being children they have forgotten them, on putting code in 3 times incorrectly it asks if you are really that person, on anwering yes it asks you to confirm your height, being children they have grow since user was set up and I cannot guess the correct height. Does anyone know how to overcome this problem without deleting all user details?

I had a similar problem when staff members blocked up the system on our rehabilitation unit.
You can either sit and patiently work through the passwords and try a different height each time it prompts you. Or you can uninstall the software and then reinstall it, although this means you will lose all data saved. I contacted Kyle Orland who wrote Wii for Dummies, and even he didn't know of any other method.
Hope this helps.