"I'm just a little girl, why would I pay taxes?"
Translation:אני רק ילדה קטנה, למה שאשלם מיסים?
Yep, ילדונת and ילדה קטנה is the same thing, just like כף קטנה and כפית, or מפה קטנה and מפית. For the other question, you can say it in the present: למה אני צריכה לשלם and also in the future: למה שאני 'אעשה'(future) את זה Though I think the former example is more right to say.
רק actually means "only", as in "I'm only a little girl and NOT YET an adult" which is equivalent to "just a little girl" in English. סתם means "just" as in "I'm just a little girl and NOT a unique little girl, like a child prodigy."
Since the context of paying taxes implicitly distinguishes the girl from an adult, רק is more appropriate. The translation should have said "only" for clarity.