Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [teacher's name],
Please be advised that due to [reason for diet], we have a special diet for [name of child]. Because of the dietary requirements, we will be sending [name of child] to school with a pre-prepared snack or lunch.
At any time should the class being having food that is not planned in advance, we would like to request that you remember that [name of child] has special dietary needs. We understand that there are parties and functions during the year during which the children receive treats. Could you please notify us of these types of events in advance? We will provide you with foods that are appropriate for [name of child], so that [he/she] may be involved with the festivities as well.
Thank you so much for helping with this.