Mavericks, Minecraft, Mods and Parental Controls
You'll have to open up Terminal.app for just a minute. This is what you should enter:


sudo chmod 777 '/Users/sandbox0/Library/'
sudo chmod 777 '/Users/sandbox0/Library/Application Support/'
sudo chmod -R 777 '/Users/sandbox0/Library/Application Support/minecraft'


Obviously, you'll need to change out sandbox0 for your son's name, that was just the account I used to test it.

So what do these commands do?


sudo means you'll be executing the command as root to ensure you have full permissions. You will need to enter your password the first time under a nice, big warning.
chmod means “change mode” which will change the permissions of the file, with the Library folder, the Application Support folder, and finally with everything in the minecraft folder. (the -R means recursive, going through all the files and folders inside that one).


You may have to run the final command once in a while as new versions of Minecraft are downloaded and ready to be modded; a new JAR that Minecraft downloaded on his account will not allow you to edit it. If you will be doing this often, open the Applescript Editor, paste this in, and save as application.

do shell script "sudo chmod -R 777 '/Users/sandbox0/Library/Application Support/minecraft'" with administrator privileges