In XP, you can use the net user command at the command line to restrict when a user can log on. Of course, your daughter will need to have her own user name and password and you'll need to set accounts and passwords for everyone else that she doesn't know, so she can't simply log on as someone else.
For example, if her username were mary, to allow her to only log on between 8 a.m. and 8 p.m. on Monday through Friday, you would type the following at the command prompt:
C:\>net user mary /time:M-F,08:00-20:00
To get to the command prompt window, in the Start; Run box, type: cmd.exe
This keeps her from logging on after 8 p.m., but if she's already logged on, it doesn't force her to log off.