@strum
It's not the mask that is a part of an actual full costume, so she isn't partway there. It's a standalone mask; one of those half-face ones with an elastic band that used to be popular in kids's shops.
There's a way to tell.
The #Batman symbol never goes on the head. In-story, there's a fairly credible reason that the symbol is bright yellow and always on the heavily armoured chest. People making full costumes follow that design, and you'll never see the symbol on the mask in such a costume if it's based upon the actual character.
Pre-Keaton Batman full costume had the underpants-over-trousers thing, moreover, which I cannot see working well for a little girl.
Interesting that she's definitely holding a Batman, not a Batgirl, mask.
That palm tree logo on the T-shirt is ringing a faint bell, but I cannot immediately place it.