Here are a few suggestions:
* Walk toward him, saying "Dog!" and then praising him quietly and warmly. Over a few weeks, work up to running up to him while saying "DOG!" loudly. Make it a game that always ends with warm praise and petting. Do not use an angry voice, have fun. If the dog backs away, ducks away or in any way appears stressed, move more slowly, speak more softly, praise more enthusiastically and give him some delightful treats.
* Make some recordings of children making noise and play it daily when he is eating. This way he'll associate something pleasant (eating) with the sound of children.
* Whoop it up when you play with him. He will soon think loud sounds mean good things. (If he gets too excited, stand still, be calm and have him lie down. Leave a lead on if you need to so you can manage him better. Your puppy needs to learn excited sounds don't always equal excited actions.
* Generally make more noise. Drop a pan, slam a door, toss a few magazines on the floor (not at the dog please). Children = noise so the more he hears now, the more accepting he'll be of it later.
* Read our books Childproofing Your Dog and My Smart Puppy for more exercises and games that will help your dog be the best dog for kids possible.
If your dog seems frightened or confused by any of this (or, if you are), please find an experienced pet professional to give you hands on help.
Reminder:
As with all of training like this, please do it out of sight of children. We do not want your children imitating you.