Sun City is great fun. Like a Vegas, but smaller and I think classier.
There is a fantastic artificial wave pool and beach.
Very close to SS is Pilansburg game reserve. You can drive in on your own, and they can organize tours for you. They have all the big animals you will want to see.
http://pilanesberggamereserve.com/index.html
There are a lot of options from rustic tents to Tshukudu (up market - game drives included, etc.) Your 4 year old will have an excellent time, and husband will also have great fun.