How old are they? How do they try to play with him?

You need to work on this. Keep your dog on the leash and with you. Make him sit, praise him for that, then let the kids come to you. They need to not act afraid.

When they approach, make sure he stays sitting. Praise him for this. Then they can pet him. It; a first step.

Has your dog been to any obedience training? How old is he/she?