There are numerous reasons why a parent would need to write a letter of permission for their child. Generally, it is for something like a school field trip, though it may also be for someone else to pick your child up from school or for him to engage in a special activity. Whatever the reason, it is important that your permission letter be clear and concise, so that the receiver is clear that you give permission as your child's legal guardian.
Address the person to whom you are giving permission. In other words, if you are giving your child permission to do something school related with a teacher, then address the teacher in the letter. For example, it might read: "Dear Mrs. Williams." This is important because you give the permission to the teacher specifically, instead of just anyone involved in the activity.
Write about the activity or event specifically. Also, be sure to name a date, so that there are time parameters on when your child can participate. You aren't just giving the teacher general permission to do whatever with your child. Be specific to the activity at hand. For example, "I give you permission to take my son, John Smith, to the school-sanctioned carnival, along with the rest of his class on April 1, 2011."
Include stipulations if there are any. This kind of clear communication avoids mishaps during the midst of the activity or event. You might say: "John is not to eat sugar of any kind while at the carnival, as it aggravates his diabetes and may cause a medical emergency."
Provide your contact information at the end of the letter. It may read as such: "If there are any emergencies or problems during your tenure of care of John, please don't hesitate to contact me at 777-777-7777 or via email at firstname.lastname@example.org. Then, end with a thank you and sign your name.