1) First of all, set your camera to aperture priority mode (AV) or Manual if you know how to do this.
3) Choose a small aperture - for these photos I used F2.8 but you can go lower if your lens stops down further. The lower the number the better the bokeh.
4) If you are shooting in low light you will definitely need to bump up your ISO. I needed to use ISO6400 for these shots, the maximum for my camera but I was shooting at night - if you do this during the day you won't need to go this high.
5) Depending on how much light is in the room, you may also need to use a tripod or just rest your camera on a flat surface. If your shutter speed is under 1/60 then you will need to do this, (I just used a coffee table as I was too lazy to go get my tripod!)
6) Focus on something in front of the tree - I used my glass of wine, but you can use anything you like!
7) Take your picture!
You can do this with your kids (having them sit in front of the tree) but you will need more natural light than I was using - it will make the whole process much easier. (Even with my ISO at 6400 and my Aperture down at F2.8 I was still only getting shutter speeds of around 1/20. This is too slow for handheld shots but also far too slow for kiddos to stop blur from their movement) Callum refused to pose for me, so I choose I moment when he was relatively peaceful watching TV to give it a shot......
However it is much easier to do this during the day so you can get a higher shutter speed. It's exactly the same process.
The following photo was taken in exactly the same way too, the only difference is I focused on an item in front of the lens (I just held up a card in front of me) then removed it before pressing the shutter. Provided you don't try to refocus, you will get the whole tree blurred and with the nice bokeh circles. Alternatively, you can change your lens to manual focus, and unfocus the frame until you get a shot you like.
I also found this article over at Digital Photography School that has some amazing examples of how people have used this effect creatively so it's worth checking out for some inspiration.
Just some ideas for you to play around with this Christmas!