If someone does figure out the cause of your primary issue (see 3rd paragraph for a possible solution), you also have to make it possible for your son's account to write to several folders and files in the Planeshift directory. The easiest solution if you have the disk space is to create a folder called Applications at the same level as Documents in his account and put a copy of Planeshift in there; however, don't do that from your account, log into his account and copy the planeshift folder to his Applications folder.
This brings up an issue to examine, there may not be enough identifiable characteristics of psclient for the limited account system to identify it properly or perhaps having more than one on the system at a time could be issues.
Being a long time Mac user (512KE anyone) and a Unix user (SGI Indigo2 with IRIX) I have found it best to set permissions from the terminal, they don't anyways work the way I expect using Get Info. "chmod -R ugo+rwX Planeshift3D" would give user,group,other (owner,group,world) read and write permissions to all files and folders in the directory Planeshift3D, in addition also give execute permission for all three for anything that already has it for one of them (rough translation).