It appears that you and you son are both playing the game on the same Windows user account, which is why you are having this difficulty - both you and your son are using the same file location for your savegames. The savegame files are stored in My Documents\LDW\Virtual Families, not in the application folder. The savegame files are technically user files, and to be compatible with later versions of Windows, the savegame files must be stored in My Documents.

Since booting back and forth between Mac OS X and the Boot Camp partition is a very time-consuming process, the easiest way to resolve this issue is to create a separate user account on Windows for your son. That way, his games will be stored in his My Documents folder, and yours will be stored in your My Documents folder (e.g. C:\Documents and Settings\Mom\My Documents\LDW and C:\Documents and Settings\Son\My Documents), so that your games won't be interfering with each other.

To create a new user account for your son, go to Control Panel and select User Accounts. Just follow the instructions there for setting it up. There are also step-by-step procedures in Windows Help.

To save space, you can delete the second copy of the game, if you like, since both user accounts can run from the same application.