Trip to Flinders/Gammon Ranges Dec/Jan

IMHO I would go to Coffin Bay even though it is more crowded at that time. Large areas that may have bush fires is not a good place to take 6 kids.

We camped in the Flinders Ranges just before Christmas a few years back. One day it must have been 45'+ I remember waking in the middle of the night terribly dehydrated. Way too hot to enjoy these spectacular NP's. We have been back a couple of times since in June and also in October a much better time of the year. Strongly advise that if you want to enjoy this beautiful part of Australia don't go in Dec / Jan. Anywhere along the SA coast that has a good beach would be better in the summer heat. There are plenty of places in SA. As you mentioned Coffin Bay, also Streaky Bay area or Port Lincoln. You have the Eyer Peninsula or the Yorke Peninsula as well or go south of Adelaide to Victor Harbor / Robe / Normanville / Port Elliott etc.
Good luck, don't forget you may need to book asap.

I’ve once enjoyed a trip to Wilpena area in December – camped at Bunyeroo Gorge. I recall after each day of hiking that we just sat in water stream with a beer – beautiful (but don't camp in a channel - we had a flash flood come through that trip in the middle of the night after an evening of thunder). I've only been to Gammon's at Easter - spectacular area but reckon that would be a bit hot / dry mid summer. I thought you could check temp’s just before going – if going to be above 35deg most days then head to Coffin Bay. Enjoy

High probability that it will be very dry and very hot...if you have the option, April to October is best for that region.....as mentioned, the coast is a far safer bet for mid summer. I think the BOM are forecasting that this summer will be harsh.

The west coast of the Eyre Peninsula can be cool when inland South Australia swelters. There is a lot to see on the Peninsula, and places like Streaky Bay and Elliston would be good to start with.

We were just up there 2 weeks ago. Virtually all creeks were dry. I would suggest going between April and October. Funny enough my daughter from Melbourne is heading across the Nullabor in December and I think that it will be too hot heading your way.
Nights are very cold in winter up in the Gammons but days are magic. Happy travels.
Chris

One early morning near Blinman in winter I got up to make my partner her usual morning cuppa. Opened the chest fridge lid and the temperature in it dropped!

Blinman area has a bit of altitude.