I admire your not wanting to let this child slip through the cracks. However, I really truly believe that calling CPS might be jumping the gun. I have never had any direct dealings with CPS, but I know a lady who is a CPS case worker. Believe it or not most of their calls are false alarms or the result of family discord and one family member calling on another for made up or over exaggerated circumstances.
My suggestion would be to go to the girls home and introduce yourself to her parents. Maybe you could bake a dish and take it as a "Welcome to the Neighborhood" gesture. That would open the door for you to get to know the parents. It could be that the girl is stubborn and refuses to use soap in the bath or wash her hair and she has that favorite outfit. Her parents may have tried everything and are now allowing her to face her peers and receive ridicule from them in hopes she will understand the importance of clealiness. It could be that the parents are young and need the advice of an older, more experienced parent to help them find ways to teach their daughter these things.
Now, if you meet the parents and feel that there is neglect or worse going on, then you must do what is necessary for the child. I would just try to meet the parents first. Also, even if you do call anonymously, EVEN if you do block you phone number when you call, they STILL receive your phone number and with technology these days, you are not anonymous. My friend has told me about cases where they mistakenly call the wrong number back and therefore reveal the anonymous (to the suspected parent) reporters identity. So if you do call, go to a pay phone if you want to truly remain anonymous.
L.