Some organizations/schools will provide support for dependents. This support can include housing, airfare, and paying the tuition for the international school your children will attend.
Many international schools will often accommodate a teacher’s husband or wife and their children to accompany them, but this does not normally include other relatives such as parents, siblings, or grandparents. They would each need to arrange their own individual visas, which normally require securing employment in the country in advance, in order to live there.
Sponsorship for dependents is normally valid from the start date of your contract and expires at the end of your contract, regardless of age. In this case, your child should be able to live with you until the contract ends even if they have reached the age of 18. If you were to extend your contract, your child’s age means they would no longer qualify as a dependent and would need to make their own visa arrangements if they were to remain in the country.
Review the "Job Benefits" section of any job listing to find out if a school will provide dependant support. If this information is not available in the job listing, ask your Placement Coordinator or the school's hiring manager during your interview.