How overweight is overweight?
Girls at around this age tend to have a bit of puppy fat. I know I and my sisters all did at around 12-13 and it all started falling off when I was 14-15. Just be careful!! Weight is such a touchy issue!
f she's eating well, exercises and is happy, I'd personally leave it (unless she's hugely overweight and it's affecting her health).

Good luck with it.
She is 58kg and about 150cm tall. So she's in the middle of the 'overweight' range. It's not just now - we've been battling this since she was about 7. The problem is that she only exercises when we make her.