Honestly there's not much a 12-year-old can do in terms of work! I'm 15 and still can't find anywhere that'll hire a 15-year-old. But, she could start a small business, maybe selling handmade crafts if she enjoys that sort of thing, maybe cleaning a neighbor's house. Dog, cat, and house sitting would be a good 12-year-old occupation. I usually sold lemonade at the end of the driveway till I was 13, but of course that doesn't make too much.
If you're comfortable with letting her walk around the neighborhood, she could try walking dogs or maybe being a 'Mom's helper', where the mom is still at home but the helper keeps the kids occupied or does small chores.
There's also some modeling opportunities if you look for Children's Place, Justice, Aeropostle PS, and other tween girl stores.
That's all I can think of off the top of my head! Good luck!