@wonderbly has already provided you with everything. Below I am providing letter of expalanation which might give you idea to write for yours.
Dear Citizenship and immigration officers,
I am writing for the Letter of Acceptance for my son [NAME] for his study permit application. He is only 7 years old.
As per information about minor children before entering Canada, available at:
https://www.canada.ca/en/immigratio...nada/study-permit/prepare/minor-children.html
Minor children who want to study in Canada must apply for a study permit before they enter Canada. This includes minor children who come with parents who already have a valid study or work permit. If they’re coming to Canada with parents who have a valid study or work permit, they don’t need to provide a letter of acceptance from a school when they apply for a study permit.
[NAME OF KID] will be traveling with me during my stay at Canada, therefore, he does not need Letter of Acceptance for his study permit application at this stage. This letter is provided for the purpose of clarifications. I hope you will accept it.
Your support in this matter shall be of great appreciate. Thank you.
Sincerely yours,