Gaelic-medium education must be provided by councils where 'reasonable demand' exists and so you have the best chance of succeeding with this request if you have the support of a group of parents.
Telephone your local council office and ask to speak to the Gaelic officer (if the council has one), or to the education department. Either of these should be able to advise you to whom you should write. You should put your request in a letter, and request a reply. You could also write to your MSP.