these really arent only child issues, they are personality traits. :D
your daughter is a laid back girl. also shy, etc.
this is, for the most part, a good thing. ill bet shes also in tune with people's emotions, very sensitive, etc. right?
the only problem is, like your example, people will walk all over her.
the key is to get her to assert herself, not be more aggressive.
how you do that, i dont know.
im that girl, im the sensitive, laid back, shy girl who doesnt know exactly how to keep people from walking all over me.
so good luck with that.
www.askdrsears.com might help.... or ...
consciousdiscipline.com
see if any of those help you.
most of all support her to speak up at home with you and dad. shes old enough where she can say something to you right? especially in your home where you can help her to learn WHAT to say.
:D
good luck.
:D