Of the places I've been the most suitable are:
Queensland is my favourite place but heck of a flight but Great Barrier Reef around Port Douglas/ Cairns - we went on Seastar, cuddle a koala - Wildlife Habitat PD, tree kangaroos - we stayed at Lumholtz Lodge where lady cares for them and you can see them (one jumped on me!) and you can feed possums, wallabies, bettong etc. Also saw cassowary - 6 foot tall birds in wild. Beautiful weather around 27C everyday. Rainforest in Daintree were can see crocodiles and go ziplining and horseriding on beach. You fly via Sydney if go with BA so can do speedboat, Harbour Bridge Climb etc there but weather isn't great there - 15C to 20C in August so wouldn't hang around. Book multi flight with BA if doing this - our internal flights were almost free when did that - no idea how that works. Our flights were around £1100 each for London to Sydney then Sydney to Cairns and return, rest of costs similar to UK. Super friendly, almost everyone said welcome to Australia to us. Magnetic Island looks amazing too and has rock wallabies and koalas in wild and horseriding on beach and sea. Would happily live there.
Cape Town - see penguins, big 5 at Aquila, Table Mountain etc, whale watching (did that in Oz too) but take a lot of care with safety and weather will be quite average there so may not be the heat you are seeking.
Costa Rica is great, though more for teens. Rainforest, active volcanoes, white water rafting, ziplines (some seriously high ones a few hundred metres up - my 10 year old DS loved these but he's ASD with no sense of danger so many adults screaming and him just fine!). I wouldn't let a 6 year old on the ziplines though, again white water rafting not sure as its pretty wild there. Lots of lovely animals there - we saw racoons, coati, sloths, monkeys, toucans, etc. Would say to enjoy it all you need to be 10 plus so they can do everything and remember it though may still work. 11 hour flight. Weather was 27C to 30C each day with about an hour of rain each day always late afternoon.
We also don't go to Carribean then as its hurricane season but lots of people do. With a 6 year old though I would actually go closer to home maybe Azores to see dolphins / whales or Scandinavia to see animals in the wild like bears / moose / reindeer / beavers and some canoeing / gentle white water rafting but temps only around 20-22C.