Turn off ALL the lights except for the tree lights. You want your house to be pitch black. The only lights that should be on are the lights from the tree.
Crank up your ISO! Since you are shooting in extremely low lighting you will need to push up or even max out your ISO. Don’t stress about theta noise created by doing this. It is better to increase your ISO than have an underexposed image.
Meter off the light hitting their face. Since you have very little light to work with you have to work with what you got. This may be a challenge but you will want to meter off the light that is hitting the faces. This will insure the faces in the photos are illuminated properly.
Set your white balance. This one is extermely important. Christmas lights can have crazy colors that tend to reflect onto your child’s face. To avoid this, set your white balance prior to taking the photo so you have less of a headache in post-processing.
Tell your subject to FREEZE. Because you may need to lower your shutter speed to get a properly exposed image you do not want them to move.