Can anyone help me with sizing - child's to adults

[PurpleO]

I have a niece who is 10. She's wearing teens clothes already age 13 and 14 clothes.

I bought her a t-shirt in a women's size 8 and it fits her. It's a little long but it does for her. I'm thinking about buying her a dress. I don't know what size to go for. A women's size 6 or size 8.

Can anyone help me please. Thanks

Is a women's small fits then I'd go for a 6. As she's only a kid an 8 may come too long