When I hear, "little girl," I think of a child between the ages of 2 and 10 years of age. However, your words make it sound like you might be talking to an adult who has control over where she will travel.
When face to face with a young child, you can say, "you are so cute," or "you are so adorable," but how they take this can depend on how well you know each other and your tone of voice and body language.
"How do you like Istanbul so far?" Yes
"I hope you have had some fun here till now." I would say, "I hope you have had fun so far."
"I wish you could stay here longer." Yes
"We always meet each other in Istanbul we have to make new plan like going to Europe." I thought you said this was the first time meeting this person, so I am confused. If this is an adult, how about, "We have only visited each other in Istanbul, what do you think about making plans to visit each other in Europe?"
What else I can tell them to finish our conversation well and say good bye. Can you give me some suggestion?
"It was nice to meet you. I hope you enjoyed your time (or enjoyed being) here and I hope to see you again soon."