Yes, you can give your 12-year-old daughter melatonin if she is having trouble falling asleep at night. It can be helpful, especially if non-pharmacological measures have not been effective. Melatonin has been shown to be effective in children with ADHD and insomnia when used in combination with good sleep hygiene practices. It is typically recommended to administer 3 to 6 mg of melatonin 30 minutes before the desired bedtime. However, it is important to note that there is limited data on the long-term safety and effectiveness of melatonin. If you have any further questions, feel free to reach out for more information.