Honestly, your question is a bit difficult because it isn't very clear what your concerns are other than
I want to make sure that the subjects and the tree are in focus. What are the best settings for something like that?
Well, this entails simply using an appropriate aperture and a suitably wide lens to ensure focus. As a starting point for experimentation, perhaps try 35mm and f/8. Try not to have much difference in the distance between the people and the tree from the camera.
Doing this might raise other problems. The shutter speed might be too slow if your subjects have trouble keeping still. My suggestion would be to not worry about noise too much, go for 1/100th (or maybe even faster for kids) of a second and go ahead and crank up the ISO to whatever is needed to get a correct exposure for that. You've got a recent FF camera, don't feel like you need stick to low ISO.
On the other hand, if your subjects can keep still, then you can go for 1/40th of a second, put the camera on a tripod, and keep the ISO low.
I'm not 100% clear on the look you want though. For instance, are there lights on the tree that you want to show glowing as well, in which case you may need to balance that against the room lighting and have a much darker room to work with.
I am taking a photo with a Canon 5D Mark IV indoors in front of a Christmas tree using only the available lighting of the room. I do not have a flash.
If you are willing to get a cheap flash with decent light output, I recommend the Neewer TT560, it should be < 40 USD. It is fully manual, but that is fine. As you will be indoors, I would recommend using it by pointing it at the ceiling, this will provide much more even flash with more natural shadows (as it will seem like the light is above them). This plan could be foiled by high ceilings, or an oddly colored ceiling (can cause the bounced light to have a different color). Experiment with it ahead of time to get the light level where you want it.
We don't know what the available lighting in your room is... my living room has a big window that provides very sufficient light in the day. We don't know if you're working with 1 window, no windows, etc.. and also your artificial lights, 1 bulb? 6 bulbs? How many lumen? It is difficult to give any further advice unless we know what you have to work with.
If you wanted, you could even take the shots from a tripod and get the tree and other subjects in focus individually in different shots and combine them in post processing.