Is the hijab obligatory for a girl who has not yet menstruated and whose body is still small?

The signs of puberty for females are four: menstruation, the growth of coarse hair around the private parts, seminal emission, or reaching the age of fifteen lunar years. Whichever of these signs appears, the pen of accountability has been applied to her, and she is obligated to adhere to all rulings of the Sharia, such as complete Hijab. What you mention about not being able to bear the responsibility of Hijab is from the embellishments of Satan; for a Muslim does not abandon some rulings of the religion. Allah Almighty says: "O you who have believed, enter into Islam completely [and perfectly] and do not follow the footsteps of Satan. Indeed, he is to you a clear enemy." (Quran 2:208) And the believer's chest expands to Allah's decree and is content with it. Allah Almighty says: "But no, by your Lord, they will not believe until they make you judge concerning that over which they dispute among themselves, and then find within themselves no discomfort from what you have judged and submit in [full] submission." (Quran 4:65)