If it's stinging her when she goes to do a wee tell her to sit on the toilet and try to lean as far forward as possible while she wee's, this will make the wee run forward rather than backward over her scratch!

I would probably put some paw paw ointment on it, even if it doesn't help it heal quicker it will put a barrier on it.

Have you had a talk to her about playing with her self, at the age they start to become a little bit more aware of themselves and just start checking things out down there (not in a sexual way at all they just just get a bit curious about what's going on)..

Once or twice I have gone into the shower/bath to get DSD (6yrs) out of the shower and found her having a little a look/fiddle down there..

Hope her scratch heals quickly!