It is likely the minor applicant received a notice to attend the PI Interview and it included a notice for the test even though the minor applicant is not required to take the test AND will NOT be asked to take the test . . . just interviewed.
IRCC tends to use stock templates or forms for the notices and it is up to individual applicants to discern what part of the communication actually applies to them personally.
It has been several years since I similarly received a notice to take the test even though I was test exempt (at the other end of the age spectrum, by a lot). At the time scheduled those taking the test went into one room and a couple of us, who were test exempt, went into a separate room to be interviewed. No test.
I have seen a number of similar reports by forum participants more recently.