iphoto library sharing from remote imac

I have an external drive physically attached to my main iMac. I got my kids an iMac for Xmas. Currently, my iPhoto library is on the external drive and I would like to share it with my kids iMac so they can add and view photos from their iMac. I've turned on sharing in every place I can think of but continue to get an error on my kids iMac that says:

Caution

The iPhoto Library is locked, on a locked disk, or you do not have permission to make changes to it.

Can someone help?

Thanks,

Stacey