It sounds like a lovely thing you are trying to do OP.
I have a 10 yo Dd and totally get not wanting your niece to stand out or be targeted.
My Dd recently went to a school disco and wore a t-shirt style dress with thin straps, think of a vest top but knee length with hi-top converse and a hoodie over the top.
However she is more girly compared to a lot of her friends.
A lot of them had combat trousers and slightly cropped T-shirts.
One place I'd recommend for that age group is 915 (within New Look). It's quite reasonably priced with an extra 25% off just now (I just ordered stuff for Dd 30 mins ago).
One reason I'd suggest there is that they do such a good range from completely casual and almost Tom boyish to quite girly but still relatively casual. They have band T-shirts etc so plenty of opportunity to express their own style and they would have something to cater to most tastes.
I absolutely get PPs saying she should develop her own style but shopping and choosing your own clothes & trying things out is how this starts. It doesn't sound like your DN gets much opportunity to explore this if her mum isn't necessarily up to speed on the various options or particularly interested in fashion (not a criticism).
My Dd and her friends were definitely starting to show an interest and wanted a say on their own clothes from age 8/9.
Unfortunately girls of 9/10 can be so nasty and definitely judge each other. So I'd suggest she gets something to her own style so she can be herself but also something she is comfortable and happy in - a wee self confidence boost could be just what she needs. You sound lovely 🥰
If you're not able to take her shopping or give her a voucher to choose her own stuff, maybe she could have a look on the website for what she thinks is nice (that way she is starting to develop her own taste/style) and will hopefully feel great at the party!
One thing I would say is that even if she does fancy a dress or skirt, girls that age and right through teens would usually wear them with their trainers
www.newlook.com/uk/kids