I know you can customise the iPhone main passcode to strengthen the security but there doesn't seem to be that option when doing the same when setting up restrictions. After I have written it down somewhere I don't want to remember the passcode, and I definitely will if't it only 4 numbers. Is it possible to write an app for something like that?
Very much doubt Apple would allow any app to access that API.
The restrictions passcode is so hard you need to wipe the phone & start over if you forget it.
If you really don't want to know what the code is, get someone else to set one & not tell you what it is. You then have the opportunity of changing your mind later, assuming they remember it.