Wow, you could spend 2 weeks just in and around Cape Town!!! But there is so much else to see. For a first sightseeing trip I would suggest travelling down the Garden Route which is a coastal road taking you to the Eastern Cape with lots of places to stop along the way. There's really a lot for children (probably more for your elder son though). There's a huge aquarium in Cape Town at the Waterfront - one of our biggest tourist attractions, so a good place to go - they also have boat rides etc. Closer to us, we have Monkey Town just outside Somerset West (go in the morning - it can get REALLY hot), there's also Butterfly World which the kids love - it has birds, reptiles, spiders and a touch farm with the usual small animals, there's also the World of Birds in Hout Bay (part of Cape Town). The cablecar going up Table Mountain (like I say - a lot right here). If you carry on down the garden route, there are great beaches, I think a Monkey World and another bird place. Lots of little farm stalls to stop at and stretch legs and eat. If you get as far as Port Elizabeth, there is an Oceanarium with a fantastic dolphin show. Oh and along the way the Outeniqua Choo Choo which is a steam train which does a day trip along beautiful scenery. And you must do the game farm thing - not too sure where you'd go in those areas, but there are LOTS - pretty expensive for us locals though.
I will chat to a friend of mine tomorrow who I think has a CD with songs about African animals. You can also look on Kalahari.net (a more local version of Amazon) - they might have something good.
Please feel free to PM me if you need more info. DH is a tour guide, so he will probably be able to help more when he gets back.