biting children

I have a three year old Jack. He has bitten two children both age 6 in the side of the leg. Both of the children were bitten in our home when the entered the house suddenly. I now have no trust for my dog and when children come to play at our home I keep a mussel on him. This is very traumatic for him, he stays in the same spot until I remove the mussel. Can I correct this behaviour. He does not react this way with adults when they come to the door only children he will instantly get nasty with.

Help! We love our Jack very much.