Alternatively .. you can use the feature of iTunes to have multiple Libraries on the same user account. This saves you logging in and out on every users account just to update their ipods.
1. Make sure iTunes is NOT running .. then while holding down the SHIFT key .. open iTunes.
2. You will then have the option to open an existing Library or Create a new Library.
3. Create a NEW library for each of the 2 daughters and son and give them recognisable names.
Now each person can just open their own iTunes Library file and manage their video and music .. their way.
I recommend this free tool called Libra to help creating and managing these multiple Libraries
http://homepage.mac.com/sroy/libra/Another advantage is that there is only 1 copy of the music folder on the PC but everyone can include or exclude whatever music and videos they wish from their own Library.