Be very calm around him, pet him, and if he rolls over for a tummy skritch, then have the kids come down and gently rub his tummy. Teach them to tell him to "sit," and reward them each with a cookie (species appropriate) when he sits.

Is he good on a leash? An adult could be hanging on to the leash, take a walk and let the children "help" by hanging on to the leash, too.

In time, they'll forget they were ever afraid. Just tell them never to bother him while he's eating, or to tease him.