If you get in there first, as I understand it, you would get all of her tax free allowance anyway.
Have you mentioned the ' pocket money' is gross?
I don't explicitly state this in our AP letter because instead I tell them we would prefer an AP not to have a second job for various reasons. If they aren't happy about this, that's fine, but we'd prefer someone who was.
I think you may have to file something monthly, which could end up being an admin pain - if I were you I would also call HMRC and check, not just leave it to her. Is she a native speaker?
My friends whose au pairs have had other jobs found themselves treated like a bit of a boarding house as the arrangement progressed and jobs that started off as a couple of hours here and there seemed to expand. I'd think hard about how it would work.
It would be interesting to hear from people who made this work.
I didn't take on a girl this year whose English was way too good for classes - I had concerns she'd get bored and as our talks progressed, the working in a second job came up.