We know that some children can have concerns or even anxiety about seeing Santa. The best thing you can do is step back and let us try to work with the child (or children). Often, coming in to "sooth" them can make things worse as the child then feels that they are in a situation where their caregiver feels it is appropriate to provide comfort (like the doctors office where they get their shots...). If you can stay calm and unconcerned as a parent, even laugh and play with them and Santa together, we can often achieve a satisfactory result in the end.
Once we are able to establish a trust with the child, the best portraits can usually be made. Remember, telling your child to smile and have fun rarely works. If they see that YOU are relaxed and having fun with Santa, chances are, they will too!