If she's past her isolation date (10 days from start of symptoms OR from date of positive test for asymptomatics) she is no longer infectious. PCR tests may still come back positive as inactive pieces of the dead virus are shed in the mucosa- some people are persistent shedders but are no longer infectious, that is why the CDC moved to recommend no re-testing within 90 days of infection (that and its extremely unlikely to be reinfected that early). Certainly check with your pediatrician, but don't base your decision on misinformation.

I'd be relieved my nanny already got covid. And vax'd. Double immunity right there.
Won't be any issue from here on out.

Fyi - you still test positive for covid weeks after quarantine. You're not contagious, obviously, but there is residual virus in your system that triggers the lab tests to reveal positive.

Good help is hard to find. She told you she had Covid which is also saying something. If you think it will be easy to replace her then go for it. If you found her desirable enough to hire then someone else will too.

Take your chances with someone else. I’m all about self preservation so if it were me I’d take you money, plus the 25% use it to keep the lights on while I looked for less cheap and more compassionate or at the very least less shortsighted employers.