"Your girlfriend is eating soup."
Translation:Твоя девушка ест суп.
Девочка - little girl. Девушка - young lady or, with a personal pronoun such as моя (my), твоя (your), его (his) etc. - a girlfriend (although still subject to the age constraint).
"Твоя девочка", on the other hand, will most likely be understood as "your daughter" (unless you are still in your early teens).
Isn't it true that девушка also means a waitress? And therefore, couldn't the sentence also mean 'Your waitress is eating soup'?
I've never heard of this use of "девушка". Perhaps you are mixing it with using "девушка" to address a young woman whom you don't know by name, similar to addressing someone as Miss (or less age-centric Madam) in English? As such, I can easily see someone addressing a waitress as "девушка", but it certainly would not be a substitute for her profession.
Кушает - don't use this word unless you really know what you are doing. This word has historically acquired a flavour of being overly servile (it had been used by serfs in reference to their masters' eating). If you know what you are doing, you can use it while talking to a small child or politely inviting guests to join you at a dinner table.
In any case, under no circumstances should it be used in either first or third person (unless you are trying to mock someone).
I suggest you read the post by szeraja_zhaba on the typical word order here: https://www.duolingo.com/comment/11607192
Given that the useful bit of new information is typically placed at the end of a sentence, your word order suggests the she eats soup (while everyone else uses it for something different).
Not the ordering but rather the intonation makes such a modification. Although the ordering can also matter since by default the last word is stressed and thus definitive.
Russian is generally an SVO language: subject-verb-object, so usually you should prefer the same sentence ordering as in English, although in the colloquial speech it's often fairly mixed. Your variant sounds more awkward but still understandable.
You are using the wrong first letter. In any case, "есть"="to eat", it's the Infinitive form of the verb. On the other hand, "ест"="eats" is the present tense, 3d person form needed here.
I used дедушка. Not девушка. I could swear it was a word I learned in another lesson. :(