You might have issues with your 3 yr old holding still. You'd need a fairly long shutter speed to expose him correctly. There is a way to do it and get a fairly fast shutter speed but you'd have to go wireless.
Expose for the candle (i.e. find out how long you need the shutter open to get a nice candle shot). Set up a flash with a snoot or grid on it to the side to hit just his face. Take the picture and tinker with the f/stop or flash power until you get his face exposed the way you want. You can always use someone else as a stand in to figure this out so the longest he'd have to sit there is a few seconds. I'd personally tinker with the f/stop and have the flash set to about half power but that's just me.
Another thing you could do to make it look cool is set up random Christmas lights in the background and try to capture that as well.
Good luck and let us know how it goes.