Be creative with permissions.
First, let's assume the kids don't have admin access, and I hope that no user on the machine does, you shouldn't be using it as the default.
But let's assume there are three different USER accounts :
Parent (your account that you use)
Kid1
Kid2
You can take your VM images and put them in a directory that all three users have read/write access to. Maybe something like /users/shared/vmimages/ or something like that. cmd-I to get information, and then make sure that "everyone" has read-write permissions (you'll have to authenticate with an administrator account).
Then copy the images to that directory (let's call them imageall, image1, image2, image12, imageme)
Next choose what image you want to set permissions to. Let's give everyone access to "imageall". Select the file, cmd-I, then at the bottom, make sure that "everyone" has read/write access. If "everyone" doesn't exist, just click the "+" and add it. Again, you should have to authenticate with the admin account name and password.
Repeat for each image, in this example, give Kid1 access to image1, and kid2 access to image to. Just cmd-I while highlighting image1, then click the "+" and add just kid1 to the permission list, and then give that user read/write access. Also if you want to have access, give "parent" the same read/write access. Repeat for image2 but this time with kid2. Then for image12, give just kid1 and kid2 access, but not parent. Then for imageme, just give yourself access and no one else.
Now you'll have the following :
imageall : admin - read/write, parent (me) - read/write, everyone - read/write
image1 : admin - read/write, parent (me) - read/write, kid1 - read/write
image2 : admin - read/write, parent (me) - read/write, kid2 - read/write
image12 : admin - read/write, kid1 - read/write, kid2 - read/write
imageme : admin - read/write, parent (me) - read/write
Everyone will have access to imageall. The admin account has access to EVERY image. You, the parent account, will have access to everything except for image12. Each kid has access to their own image (1 or 2) and you'll have your own imageme that no kid has access to.
I know this could be confusing, but once you get the hang of it, it isn't that difficult. Just remember, that everyone needs to have read/write access to the directory, so if you get stuck, maybe make the directory simpler such as /images and then select the directory and set it's permissions to "read/write" for everyone. If you do as suggested earlier, making it /users/shared/vmimages it will work but you have to set the permissions for the entire path (/users should already be set).
Hope this helps, and if I'm overly confusing, then let me know and I'll try to make it a little easier. I know I'm being wordy today.
