Unauthorized spend limit increase on child's data SIM

On 9 Jun 2019 I took out a 10MB Top Up SIM for my 6 year old son at R49.50/month. All was well until Jul 2022, when Telkom migrated the package to a postpaid with a spend limit of R1500. Unaware of this I was shocked to be debited R1549.50 in Aug 2022! I have since migrated the package back to a top up and tried to open a case for the unauthorized spend limit. Telkom refuses to open the case because they sent the SIM an SMS confirming the change. This SIM was in a MIFI unit, how was I supposed to see the SMS? I consider this totally ********* and ********** and don't believe I should be liable for the R1500 data usage.