I don't know if you have a Burlington Coat Factory near you, but they have a ton of dresses there. Most of them are $30 or under. We bought Elizabeth a Cinderella brand one with a matching doll dress (but it is black and goldish). If the boys are wearing navy, you could also put her in silver, but I bet she looks gorgeous in blue.
I love the idea of something like this then having the younger boys in faux tie shirts (dykwim?) and Jake in a polo, or all the boys in same style polos in different colors.
But I'm not really very traditional (*coughatallcough*) when it somes to holiday styling.