Karen McManus is an author whose works might interest your niece
https://www.waterstones.com/book/one-of-us-is-lying/karen-mcmanus/9780141375632
https://www.waterstones.com/book/one-of-us-is-next/karen-mcmanus/9780241376928
as is Holly Jackson
https://www.waterstones.com/book/a-good-girls-guide-to-murder/holly-jackson/9781405293181
https://www.waterstones.com/book/good-girl-bad-blood-the-sunday-times-bestseller-and-sequel-to-a-good-girls-guide-to-murder/holly-jackson/9781405297752
I've done a bit of creative writing from time to time, including getting some stuff published, so here are a couple of tips from me:
1. Begin a story where the action is already under way, rather than with lots of dull facts.
e.g. A reader will quickly get bored if you start a story like this:
"Melanie was 13. She lived in a small town called Marton. She was small for her age, with brown hair"
. . . whereas if you start like this, you'll be more likely to grab the reader's attention:
" 'Whose idea was it to come here, anyway?', said Karen
Melanie looked away from the rain that was pouring down outside the café window and turned to her: 'Don't blame me', she replied, 'the weather forecast said it would be sunny today'.
2. Try to provide a bit of background information about
all of the characters in your story, even if they only appear in it on one occasion.
e.g. this is fairly boring:
"Stephanie went into the shop and asked the shopkeeper whether he sold dinosaurs"
. . . whereas this adds a bit more substance to things by providing a backstory for the shopkeeper (even though this is the only time he will feature in the story):
"Mr Brown was not in the best of moods but that wasn't unusual; he was rarely ever in a good mood. He'd been running his corner shop for over a quarter of a century and, as Mr Brown saw it, things simply got worse every year.
It wasn't the competition from the big supermarkets which really annoyed him though; it was the stupidity (as he saw it) of some of his customers. They always seemed to want things that he didn't stock. They'd ask for firelighters in August and for barbecue charcoal in January. They'd ignore the small selection of red nail polish that he stocked and demand that he supply them with some that was black or bright blue. They'd expect him to have obscure medical items that even the local hospital probably hadn't got or magazines that were so specialised that W H Smith's didn't even stock them. In short, Mr Brown was thoroughly fed up of customers who asked him for items that no reasonable person could ever expect to find in a small corner shop.
It was then that Stephanie walked up to his counter, wearing her sweetest smile. 'Surely', thought Mr Brown, 'such an innocent-looking child can't be about to ask me for something I haven't got'.
'Do you stock dinosaurs?', enquired Stephanie.