Based on our conversation, I’ve sketched out some options to consider. Some places are already booking up, and availability not guaranteed on these. However, we can usually find something similar. If your friend’s family joins, it may change what options we consider. Prices listed are ballpark per person based on published rates before taxes and other fees. Because it is holiday season, they are more expensive than normal.
I focused on South Africa and Kenya but I also threw in a wild card idea at the bottom.
The mix of culture and wildlife make Kenya my top choice for you. Most of the showers should have abated by late December, leaving the landscape lush and green.

Every Kenya trip starts with at least one night in Nairobi because international flights get in late. However, it’s worth staying one more day to visit the Giraffe Centre and David Sheldrick Wildlife Trust with your kids. I really like something like Karen Gables in Nairobi, which have a nice location and reasonable price – starting at $400/nt.

The Maasai Mara’s plains are packed with wildlife and the reserve is more quiet than in peak months. It is often booked as a finale to your trip, but sometimes I start here, so you can check all of the animals off your list and relax later in the trip.
Availability is also good at Tangulia Mara on your first few days too. This is one of the few Maasai owned camps in Kenya, so the guiding is great. It’s not luxurious but it’s comfortable and the price is great for the Mara – starting at $855 per adult per night and $555 per child per night (age 5-15) – not including park fees.
Availability is already limited at the safari villas I book in the Mara but I’m continuing the search.

For a more authentic experience, I think you should spend the second half of your trip further north in the Lewa Conservancy or Samburu Region. Here you can have more authentic cultural encounters and do other activities like horseback riding, biking, or walking.
You will likely spend less time on safari in South Africa, but your money will go a lot further here because of the diversity of camps and exchange rate on the Rand.

I often spend a night in Cape Town to decompress after the international flights. There is good food and plenty to do. As you mentioned, it would be worth taking the kids to see the penguins at Boulder’s Beach or Betty’s Bay. I won’t worry too much about hotels here, because there are a bunch of options ranging from $200-2000 per night.

The Eastern Cape will be rain free in December while Kruger is getting soaked. The area is composed of private reserves around Addo Elephant National Park, and in the past 20 years has become a respectable area to safari with good numbers of the Big 5 – especially rhino. I would do at least 4 nights at Kwandwe, Shamwari, or Lalibela and extend to Samara if you want a few more nights.

December is a great time to cruise the Galapagos, one of the premier wildlife experiences in the Americas. I suggest a couple of nights in Quito to see the world heritage sites before taking the 2 hour flight to the islands. Four day three night cruises start at around $3,500 per person and go up to eight days. These can also be combined with land tours of tortoises etc.
You were right that December is the dry season for Thailand, so that could be a good option. I like to build an itinerary around a quality camp in the Golden Triangle with ethical elephant experiences. However, Four Seasons Tented Camp Golden Triangle does not allow kids under 10, and Anantara doesn’t have availability. However, we could find you a beach resort if you wanted to mix culture and beach.
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