I have wanted to post on this thread for ages, as I was an avid reader who read pretty much anything which passed my nose! But I hope that I won't be stating the obvious too much.
I think that it is fine and good to want to distract by providing other books or give an outright ban, but the important thing is to explain why to dc, explain your thinking, and make it an ongoing open dialogue and negotiation - whether it is a ban or a distraction situation. If she is presenting as emotionally half her age, the talking and dialogue will help with her emotional development, but I agree with your intuition, and there are a lot of really well written inspiring books suitable for 8 year olds which would be easier to process emotionally.
I think that where you have an avid reader who reads everything, another good thing about having the ongoing dialogue and what you think vs what they think, it helps them develop a filter, and means that hopefully when they come across books which they realise may not be ok to read, whether adult or sex or just something which has been talked about, they are more likely to tell the parent - whether before or afterwards! The ongoing dialogue also helps them develop judgement about other things obviously - make choices, make value judgements, so on and so forth - so this is all good. It is a part of the wider "not for children" conversation applying to books, tv, video games, etc. And again, apologies if this is all stating the obvious.
I think that where there is trust and mutual respect between dc and parent, dc are appreciative of being protected and are accepting of it, even if they question and push boundaries - but obviously that usually takes a lot of time and practice to build with a recently adopted older child.
My own personal view of the dramatisation of TB (I haven't read the books) FWIW is that it makes dumping ground into some kind Grange Hill variant, and it is nothing like the reality, and also that in relation to Dustbin Baby not many Aprils find a lovely Marion in real life. But at the same time, I can see from this thread how the books can be helpful for some dc. I also personally found What Katy Did surprisingly traumatic, even though it was seen as a perfectly "ok" book... the only book I read as a child which spoke to me in relation to my past vs other people's past in a positive way was No End To Yesterday - it is one of the books which stayed with me, and it is a really interesting book, I think, but I'd recommend you read it before giving it to dc as it has some difficult and not very positive themes in it - and not for now, obviously.