A few thoughts on shooting in low light situations: To begin with you will need to use a high ISO. Most cameras' lowest ISO is the native (default). This is 100 ISO on Canon and usually 200 on Nikon. You may need to set yours as high as 1600 or 3200 ISO to capture the shot. Do some test shooting before your important shoot.
When traveling during holiday season, many people don't want to bring a studio's worth of gear with them. But taking pictures in low light won't always give users the most pleasing results. Here are a couple of workarounds to get better photos while carrying less gear.