NPC sims keep dying from staying in cold

If you'd like, you can change the settings for seasons. In Game Options, click on Seasons, there you can change the season length, rain and snow options, and if you want to enable or disable temperature effects on sims. At least that will keep them from dying in cold weather.

@ChildzPlease, in addition to the suggestions that @LuminaryFairy418 gave you, you have two other options:

1) you can go into the yoga studio from the map or from Manage Worlds and click on it and then choose the Build option (the round button that looks like construction tools ) to enter that lot and place the thermostat that you normally place in homes (found in the Sort by Room section of Build/Buy Mode in the Kitchen section and click on the smoke alarm icon and you will have the option to add the thermostat).  Then, when you play the yoga studio, you should have the ability to set the thermostat for the studio to Warmer. 😉

2) You can also get a bee box and harvest the honey from it everyday (they won't produce honey in Winter so it is best to stock up during the other 3 months!).  Then, if you start to get the Freezing moodlet, you can eat some honey and it will save your Sim from Freezing!🙌  (Honey will also save your Sims from overheating in the Summer! 😎 ) 

If you don't mind using mods, there is also a few mods that adjust "death by freezing/overheating" out that you can find. We can't link mods in Answers EA, but you should be able to Google them easily enough.

I'm aware of a few that completely eliminate it, and some that reduce it to a percentage of "possibly dying."

- B