Excessive rebooking fee

We booked and paid for flights ...RSA to USA in May 2020 ( 2 adults 2 kids ) Canceled due to Covid The airline promised to keep our tickets for future rebooking Rebooked out tickets in July Both kids are now > 12 yo ... regarded as adults I do understand the additional fee , but they made me pay an extra R36500 for the kids - although the costs between 4 x adult vs 2 adult plus 2 kids now only amounts to about R10000 No telephonic/ email communication available to discuss my unjust treatment .... please help !