We took a 9 and 16 yr old last summer. Not sure I would go up to the Kruger as that's a malarial area - if you stay around the Cape then you can still do the Big 5 and not worry about anti malarials.
Lots of things to do, we covered Cape Town to Port Elizabeth:
Cape Town - Table Mountain, aquarium, beaches, Cape of Good Hope, penguins at Boulders beach, the Waterfront. We rented an apartment at the Waterfront Village for a few days.
Hermanus - whale watching, lovely coastal walks. Stayed at Whale Coast Hotel, which are SC apartments, excellent.
Plettenberg - lots of things for small children here, elephant sanctuary, big cats, monkeys, worth spending a couple of days here. Tsitsikamma with the rope bridges is quite stunning.
Oudtshoorn - Kango Ranch and Kango Caves, ostriches and crocodiles.
Port Elizabeth - Addo Elephant Park is amazing, you will be overwhelmed by elephants by the end of the day, and the accommodation and restaurant at the park are good. Book accommodation through SANParks.
There are lots of private game parks throughout the Cape, many will have the Big 5, and they range from budget to luxury, so it depends how much you want to spend.
PM me if you would like more ideas or accommodation ideas.