AIBU to think ethical clothes for kids are very pricey

[cheeseycharlie]

11 YO DD is very engaged with environmental sustainability, is vegetarian and general Greta-fan. She is out of clothes having just gone up a size, so I offered to take her shopping this weekend, and she asked if we could get bamboo clothes because they are more sustainable.
I have looked online for outlets with sustainable clothing (bamboo or otherwise) for kids/tweens and everything is horrendously expensive. She's still growing and it will cost me a fortune to kit her out in this stuff, and it will last her probably 9 months, 12-15 if I buy big.
I understand that unsustainable clothes have artificially deflated prices, so maybe our price expectations are off. However the sustainable brands seem to really pitch themselves at what I consider a super-premium level (like the people who buy all their kids clothes including tshirts at Jigsaw and the like). There's nothing at a more department store sort of price point.
Is there a more cost effective way to make conscientious buying choices for her?
We have lots of second hand shops nearby but when I've looked before everything in the kids section has been a bit ratty.
Nothing in TK Maxx.
AIBU and also any recommendations gratefully received...

I second the recommendations to buy secondhand to supplement. Some are barely worn and it is essentially recycling.

Peopletree or Fairtrade but can be expensive

Skirtfanatic for reversible skirts

I'm a fan of charity shops & car boot sales & have found loads of bargains

Thankyou thankyou everyone, there are so many good avenues here that I didn't know about. I need to make a list!
We always pass her clothes on to a family member, so her clothes get loads of use. But we don't have anyone who passes down to us, so everything ends up being bought new. I already feel like it's too indulgent (and pricey).
I'll get her involved in looking through all these suggestions and planning her new wardrobe to a budget. Quite looking forward to it.
Thanks lovely ladies

The way I do more sustainable clothing is second hand off ebay. Who are by no means perfect themselves, but gives more options than what's in the local charity shops.

Another thing to do OP is agree a list and an overall budget. I don't think a pre-teen is very aware of how many clothes a person needs, so help her work that out (ie you need at least four t-shirts and two hoodies based on how often we do laundry) and then work out her budget accordingly. So she might want to spend more on a baggy hoody (that will last longer) and then compromise with a supermarket sustainable range on T-shirt's.

If nothing else, it's good for her maths!