Honestly, I really wouldn't push her to do anything. I don't consider 58kgs to be grossly overweight by any means. I know that that's what doctors and their silly little books say, but please don't go by that.
Honestly, if she's happy and eats well most of the time, has no issues herself and hasn't asked for help then I'd personally just let her go. At her age she is unlikely to put on a heap of weight she's more likely to gain height which will balance out her weight nicely. I bet by the age of 15 it's no longer a problem.

You sound like a wonderful caring Mum who only wants the best for her daughter. Good on you! I just have a bit of experience and people telling me I was overweight and telling me that I needed to lose weight has not done me any good at all and probably had the opposite affect (as well as totally beaten down my self confidence ensuring that I rarely ask for anyone's help).

BUT you know your daughter. I think if you think it needs to be taken more seriously, maybe you could talk to a doctor about your concerns.

Best of luck for you and your daughter. xx