Was in SA and Mauritius last June/July.
Mauritius was pretty but not really warm enough to enjoy the ocean so your kids may not like that.
Cape Town will likely be fairly cool,windy and rainy as it is much further south then most of the safari places you are looking at. That is their winter since it's the southern hemisphere.
Morning game drives on safari will be VERY cold usually. Think several layers to peel off as the sun rises, definitely a warm hat,gloves and a scarf for the morning drives.
Hope this helps in your decision.