I want to use the child's POV but the voice of the adult: but how do I legitimise this?

I don't think you need to formally legitimise it, as it were. Just think of the adult as a narrator whose voice tells what the child sees.

On the either-less-or-more principle, if it were me (it isn't) I'd start rather clearly in the adult voice, James-style, so we know that the narrator isn't the child they're vocalising and through whom the story is focalised (horrible word, useful concept) even though we don't need to know exactly where that narrator is standing. Then you could let it slide into a more childish voice as and when it seems right, knowing the reader is firmly rooted.

Emma

do you know if it was the editor that made Sappholit change her child's POVs? Just wondering if there is a commercial distaste for it.

If I remember correctly, it was an early version of the ms, which subsequently underwent several rewrites.
As for 'commercial distaste' I suspect it was the darkness of the underlying story that prompted the change, rather than it being told from the child's pov per se. There is a lot of commercial fiction out there told from the child's pov.
As Emma has suggested you could have a narrator's pov - the Adult version of the child - telling the child's story, while keeping the child's dialogue, but as it stands it would necessitate a major rewrite. It is not just a matter of including adult words and concepts, but also changing the sentence structure - eg. the first two sentences of the excerpt are in an 'adult pov'. Most of the other sentences are shorter and have a more limited vocabulary, much like a child's. Also you woud need to change it from the first person to the third to incorporate both povs.

- NaomiM

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....I can't see how you could keep the charming innocence of the second paragraph, where she thinks of herself as two halves, if it was told from a 3rd person adult pov.

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- it is that element of childish innocence you would sacrifice if you added the adult pov.

Read The Boy in Striped Pyjamas: a work of pure genius in terms of a child's POV (in my opinion). Everyone in our family read it and we may as well have been reading 6 different books. Our 12 year old understood what had happened at the end but not 'how', the next child up missed various adult relationships, the next one got most details but lacked the cultural capital to understand which historical characters she was meeting, and so on. I think my husband and I got all of it...

Sheer genius. I've read it twice in under a year! And a great lesson in writing from another's POV.

Hmm, this is tricky. Thanks Naomi and Emma for your suggestions. Need to read some more examples *sigh* - just brought 5 fat (research) books home from the library; when will there be *time* ??

I do feel that I need to pick a position and stick to it - at the moment I have both adult and child POV but unlike Harper Lee I'm not getting away with it! Think I will put this chapter in a drawer for a little while and come back to it with a fresh ear.

Don't forget that Harper Lee wrote the book back in the 1930's and the prose style and range of vocabulary is fairly old fashioned. I'm not sure you'd get away with it so easily in this day and age.

I second pachelbel's recommendation for The Boy in Striped Pyjamas.

Sorry to revive this thread again - I just thought of a way (very simple, as it turns out) to achieve the effect I was after, using Emma's trick of starting with the adult voice, albeit not very clearly. I would be grateful for comments on whether the following works, and if I can subsequently relax a little about any sophisticated vocabulary sneaking in...

Ah, there I am. It's May, and we are finally wearing summer uniform. After months in heavy layers, the loose blue gingham is not much more than a whisper against my bare skin. Look at me! I have slipped off my pants and socks and stuffed them into my shoes, and I'm standing with my arms outstretched in the shade of the big juniper tree, my toes curling in cool green grass and the breeze licking my body in all its hot places, as though I was truly naked. I feel like a fairy sprite freed from an earthbound spell, and I run and leap for the updrafts that might bear me away.

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I'll take polite silence as a no, then!