Hi I am from SA, that trip sounds fine, driving in SA is much easier than driving here, you just drive there is no hassle.
Durban is great, has fab weather all year round, UShaka is a must for your DS, the water park is loads of fun and and the aquarium is great.
The drive from PE to Cape Town is wonderful and so beautiful, I would recommend taking the R62 from George, it takes you off the main motorway and you get too see so much.
Definitely spend a couple of nights in either plettenberg bay or knysna, they are only a few miles apart so you can visit both very easy
There is so much to do in that area, dolphin cruise, tree top safari, you can walk with elephants, the beaches are amazing, loads of beautiful walks ...you can see I love that part of the world. 
If you take R62 route I would suggest you spend a night at Oudsthoorn, you can visit the ostrich farm and crocodile farm, both great.
Cape Town is of course fabulous and there is so much to see and, boulders beach, table mountain, cape point, the waterfront, camps bay and if course don't forgot the whine lands, spier is a must see with kids as you can taste some fab wine while your DS plays with cheetahs 
This making me want to plan a visit again.
Enjoy. You will live it