Had a thread but I seem to have lost it. Just wanted to update in case it helps others.
In short, my (then) 5 year old started developing pubic hair and smelling of BO this summer. She was referred to the paediatrician and today we have finally had all the test results back. She's been diagnosed with premature adrenarche - so the adrenal glands are producing excess hormone. There's no treatment and they don't think it should cause any problems. We'll continue to be monitored, particularly her height and weight, and have a repeat of the tests at various intervals to ensure nothing else is going on.
We go back next month and then I'll get a better idea of frequency of monitoring, but overall, good news!