I am far from an expert, but it seems like you need to determine what airline you want to fly first.
For example, we fly to Australia, so we have an American Airlines citibank card to collect miles for that since American Airlines partners with Qantas.
I looked up who flies to South Africa and there are quite a lot of options, though it appears that Delta is the only airline that flies direct...is that right?
Delta is about to change their miles/award system starting in January and it's crap. I researched it when I was looking for my upcoming trip (bc they partner with Virgin who also fly to Australia) and it's bad. So, I decided to stick with American/Qantas.
With small children, I'm guessing you are flexible with dates, which will make it a lot easier to get award tickets. It's harder when they hit school age (mine are 6 and 9).
But, once you figure out the airline options, you can look up the award charts on their websites to figure out how many miles are needed for a return ticket.
United might be the way you want to go (as LifestyleDeflation suggested), but you might want to look at their routes first. For example, I prefer flying Air New Zealand (who is partnered with United), but I don't because you have to stop in Aukland on the way to Australia. With small kids, I prefer to get a direct flight when I can...so we go Qantas.
Those are just some thoughts on the things I would consider when getting started on choosing which card to go with. For me, it depended on those several things. Hope it helps.