As I mentioned before, use pure vitamin e oil... I used it consistantly during pregnancy & now have zero stretch marks but maybe I'm just lucky?
Try not to lower her confidence over it, my friend's puberty stretch marks faded a lot with time but she always feels they're really bad because her mum made her feel that way
I also use raw shea butter and raw coconut oil. Also lemon juice is good one but because of her age I'm not sure for kids. I soak cotton ball in lemon juice and rub it over marked areas and let sit for say 10min before washing off. But please as she is 12 and her skin is younger maybe just try either of above.
My 12y old daughter has some serious stretch marks, she's using Bio oil like product ...is there anything that you would recommend?
I don't understand why Bio-Oil is so popular? If your main concern is scars & stretch marks then you can buy pure Vitamin E oil (the main active ingredient) for far less & add it to any other skin care product. I add a drop to my face cream & a few drops to my body lotion. When my daughter suffered serious burns to her legs as a toddler I used this undiluted under advice from the hospital & she ended up with almost no scarring so I swear by the stuff! Just bought a fresh bottle at a good price from Amazon: https://www.amazon.co.uk/gp/product/B00MIYAO5C/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1