I'm late to the party, and you've probably delved much deeper into genitive since your posting, but, the ки at the end of девочка can form the genitive. One of the functions of genitive is to demonstrate ownership. If you like, you can translate this sentence, rather wordily in English, to, "Where is the dog of the girl?" It is potentially most common in English to form a simple possessive as, "Where is the girl's dog?", though of course we use "x of the y" in English often enough as well. "The land of my forefathers" is an example I always think of.
Девочки, где собака? Girls, where is the dog? I won't speak for native Russian speakers, but I'd probably put the direct address at the beginning of the sentence. If I phrased it, Где собака, девочки?, I'd probably have to put a long pause on the comma to avoid ambiguity.
I think the computer is having a problem because the plural nominative form looks like the singular genitive form. Try reporting it.<pre>
Nominative де'вочка де'вочки
Genitive де'вочки де'вочек
Dative де'вочке де'вочкам
Accusative animate де'вочку де'вочек
Instrumental де'вочкой, де'вочкою де'вочками
Locative де'вочке де'вочках
So, plural genitive would be девочек .