First off wear clothing that fits the season and wear christmas colours E.g. red/green/white/gold/silver.

- http://www.modcloth.com/shop/party-dresses?price=17,70&sort=newest&page=1&omcid=0000672555&utm_source=google&utm_medium=cpc&utm_campaign=Dresses+-+Party&utm_term=party+dresses+teens&utm_content=sHSs4tAJC%7Cpcrid%7C9774079578%7Cpkw%7Cparty%20dresses%20teens%7Cpmt%7Ce#?price=20,70&sort=price%20asc&page=1
(they have sizes available in small, very cute dresses, a little high on price though)

- http://www.tbdress.com/Cheap-Women-Dresses-100507/
(oh these dresses are absolute gorgeous! extremely cheap! like $15 cheap! I highly recommend this site) GO HERE QUICK THERE ON SALE!

These are just the two links I could think of. Hope this helped in any way :)
I can't wait for what others have to suggest it gives me ideas too

Well Forever 21 is still the best for low prices, but a lot of their dresses come in S-M-L rather than numbered sizes. Sizes are weird anyway because a size 0 one place is like a 2 someplace else. This is one time it's easier to buy clothes as a boy because the sizes are in inches. The best thing is if you have a store in your area to go try some on. Here are just tow I picked out, but there are tons more at this link

http://www.forever21.com/Product/Category.aspx?br=f21&category=dress

The other store has cute dresses at low prices is H&M but I don't think they have real small sizes

My BFF Jenn buys a lot of her dresses at Charlotte Russe. So check their sire out.

http://www.charlotterusse.com/thumbnail/Clothes/Dresses/pc/2114/2133.uts

And Here are two from modcloth that I think would be great for Christmas. Their site is http://www.modcloth.com/shop/top-rated-dresses

Oh I'm so sorree but the two from Forever 21 never showed up. Here they are

dreamv on rakuten (you'll find reviews all over and on tumblr if you put it in the search). Shipping is done by the total amount , for example if you buy for less then 5,525 Y, your shipping will come out to be around 20 from Japan to the US. It is reliable , I'm actually buying a bag from there lol.
Link: http://global.rakuten.com/en/store/dreamv/
They cater to 4 different styles, princess (which is mainly girly florals and pinks/pastels ) , retro (mostly classy) , glamourous (mainly sexy styles), and an older adult style . I linked you most dresses from princess and retro.