I don't have kids myself, but my little cousin went through something very similar when she was about 11. The way her Mum dealt with it was to take her shopping and let her pick a few of her own things, so she has a sense of identity. Appreciate with covid at the moment that's not possible but could you do a little online shop with her? Doesn't have to be anything expensive, plenty of Supermarkets do quite trendy clothes for 8 year olds now. Choosing her own clothes might help her feel more comfortable. She also sat and did a little "pamper" evening with her where she let her put make up on, paint her nails, she even let her try on her wedding dress!
I'm sorry to hear she's feeling this way though, as an adult we all know how horrible it is to feel uncomfortable when looking in the mirror so for a child it must be awful.
I'm sorry to hear she's feeling this way though, as an adult we all know how horrible it is to feel uncomfortable when looking in the mirror so for a child it must be awful.