The best thing you can do is treat them as a normal child, and foster a place of safety and happiness. I have observed that children can often match the energy that adults have, in a mimicking type of way. Smiling and having fun with work is a great approach to help them break out of their shell.

I would find another student who is very friendly and ask them to sit next to and make friends with the other student so they don't feel alone and always have a friend in class.

If you have multiple students who are in the same situation, pair them up! When I have talked to people who have gone through the ESOL program, they all agreed that what helped them to have the most success and feel most comfortable is when they have community that understands what they are going through.