Remember to take the lens cap off!
Good luck! Keep in mind you have instant image review. Keep calm. Maybe give yourself a little bit of an "ISO cushion", by elevating the ISO level up three or four or five 1/3 stop clicks from lowest...makes the flash carry farther...if you need to photograph Christmas lights, the elevated ISO values help the lights "burn in" more so than if the ISO is kept at lowest possible levels. Take a few minutes to think about the settings you will be using, and to take a few test shots, and to get the exposure and lighting really dialed-in, and then shoot the shots after you have made several test shots and reviewed them and made any necessary adjustments to f/stop, flash power, ISO value, and shutter speed.
For shots of people + a Christmas tree, you need to have an ISO level of around 800, with a shutter speed of around 1/20 second, at an aperture of f/3.5 or so in order to make the lights look bright, and cheery. So, keep that in mind if you go that route.
Also, and this is important--before the shoot, you must tell the parents NOT TO LOOK AT THE KIDS during the actual photographing--that is YOUR job. MANy,many,many parents will try and "ride herd on" the kids, and will RUIN otherwise great shots by looking down at the kids. Parents really,really need to be advised that their job is to look at the camera, or at you, and NOT to talk to the kids, or to ensure that they are smiling. I'm not kidding--talk to the parents before their turns.
"Make haste slowly."