Divorce agreement questions

Hi,
I'm from South Africa.

Please can someone help. Below is an extract from my divorce agreement and i need to know when does the father of the kids have a right to stop paying for them as their is no end date stipulated in the divorce agreement:

''The defendant undertakes & agrees to pay maintenance in respect of the minor children at a rate of ...... per month per child. The maintenance will increase at the rate of 10% per annum.''

And if custody & control is given to me and nowhere does it state that the father is the guardian, does he automatically have guardianship or does it have to be stated in the divorce agreement to be legal.

SA---Maintenance can be claimed up to an age that the child is able to support themselves. However it is to be noted that generally a child becomes an adult at age 21, which means that maintenance would generally stop at age 21 unless the child is still at school or a tertiary institution or does not have an income to support them.