As far as I know there is not a lot you can do about it. If you select only your daughters or only your own playlist to sync it overwrites what is already o the ipod leavin only what you selected to be synced.
On my ipod (classic 80gb) there is a setting to adjust the master volume. Perhaps yours is set too low?
Is owning an ipod worthwhile? I have had mine 4 weeks and have reset it to factory conditions twice already!!