To make the most of the safari then we recommend that children are age 4 or 5 plus but there are excellent lodges that have no age limits on activities, so it is when you think they are ready.
As the time difference is only 1 or 2 hours and there is no jetlag it is an excellent destination for children, even for a short break.
Child related supplies are widely available in local shops around Cape Town and Plattenberg Bay.