is there any way for me to lock which applications are in the dock?
This is also built into the Parental Controls feature of Tiger.
First configure her dock the way it should be, then you'll need to go to System Preferences -> Accounts. You'll have to make her a standard account, you can do this by unchecking the "Allow user to administer this computer," and the user status will change from "Admin" to "Standard." If she's the only user on the system, you'll want to create a new administrator.
Once you do that, you can select the user, and then the Parental Controls tab and click the check box beside "Finder.app & System" and then you can configure it by clicking "Configure...". By checking all of the checkboxes aside from "Modify Dock" and "This user can only use these applications" you should be set.
The user account status will change from "Standard" to "Managed."
Alternatively, you can set these settings in NetInfo Manager, even for an administrator, but OS X throws a funky login message when you do so. You're not allowed to set Parental Controls for an administrator through the GUI, but again, it is possible as demonstrated in my screenshot.
