My sistors toy piano acide

Here is what happend the batteries that were in it leaked and you can see some white crust. I don't know how safe it would be to just put new batteries in it. It uses 9v worth of AA bateries. Is there any way to clean it and make it safe? It still works but I am worried about the new batteries leaking like the old ones.

The batteries leaked because they were bad quality or old. Just scrape off the white stuff and put in new ones, everything will be fine.

birdman is perfectly correct.
I'd recommend cleaning off parts you can't scrape off with deionized water, careful to flush and dry everything afterwards, or pure alcohol makes a good solvent/cleaner.
But that's me, I'd just go with scrape off the flakey stuff, maybe a cotton swab or two, and add new batteries, if it works you're fine. Do NOT leave devices like this on, or with their batteries in place for more than an hour.