Only one person per family can claim the CAI payment. To be eligible, you must be 18 years of age or older. If you are younger than 18, you must have a spouse/common-law partner or a child to be eligible. So a 9 year old is ineligible for the credit.
More info from the CRA: https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-re...
