I have relatives who are teachers, and they all say the same thing: the classrooms with the 9-11 year olds always reek, because there will always be at least one kid who has now got to the age where they really need to use deodorant and shower/change more frequently, but doesn't. The parents either can't enforce it, or don't realise that kiddo isn't so much of a kiddo any more.
I realise that the girl in the OP was a little younger, but the onset of puberty is generally the most common cause of this problem.
The best thing to do is for a teacher or other authority figure to mention it discreetly. So, as a parent, I'd bring it to their attention.
"Ms. Smith, I hate to interfere, but I just want to let you know that I've noticed that Susie is awfully dirty today, and has been for the past several weeks. I know it's not my place to speak to her parents, but I wanted to be sure that somebody in authority had noticed."
Then, as a parent, I'd leave it be.
Then the teacher could have a word with the parent(s).
"Bob, may I have a word with you? I couldn't help but notice that Susie isn't smelling or looking too fresh today. I wouldn't say something if it was just today, but I've been noticing this for a couple of weeks. Are there issues that I/we should be aware of? I'm sure Susie would feel better for a wash and some fresh clothes. The other children are starting to notice, and I'd hate for this to become a problem for Susie."
Of course, if the parents then do nothing, (and I definitely include working with appropriate professionals as 'doing something', if the dirtiness is a result of an extreme aversion to washing), then it's neglect, and I would call the appropriate services if I were the teacher.