In many countries it is not possible to take a child from school for this period without getting investigated by the department of education or the body in charge of schooling. This is because it can be deemed far too long of time.
Children may have to return and do a certain year in school again and at the very least should be assigned some work to try and keep them at a certain level. It can be very tempting taking school off for parents as holidays are cheaper, but it may not be wise.
A letter for this however would be:
Please excuse (pupil's name) from school for the period (start - end date) as we are taking a vacation and wish for him to be excluded from school. We understand the need for him/ her to keep up with their studies and would wish for them to be assigned some work for the period of the vacation.
I would be grateful for this and i do hope you understand. If you have any problems or questions please do not hesitate to contact me at ( insert phone number/ email).