Pre Halloween Birthday Party
Will a lot of your guests be of your niece's age? If so, you may consider Stanton Social which is very popular among college crowds. The decor is chic and modern and their food (contemporary small plates) is great for sharing.
I agree that Alta is great for group too, though I think Stanton Social is a bit more festive.
Il Cantinori (Mexican on University Place (Downtown NYC) - it always feels like a party there and it's very festive. The food is great (I love their Cheese Enchilada's), their drinks are fun and the price is right!!! It's a great place for a crowd, and the service is super; they are VERY accomodating! Your Brazilian Family will love it!!!