Hi Denise,
My boys don't do well with kids because they don't trust them. Kids move fast, they are unpredictable, and often loud. This makes my guys nervous. We have had bad experiences too with kids being rough and sadly, throwing things at my doggies.
The problem could be an inherent personality trait or it could be a bad experience. I would supervise your dog and the kids when together and enlist the kids to participate in training exercises. Teach the kids how to recognize when the dog is overwhelmed, nervous, or not ready to play. Show the kids what to do.
For visiting kids, don't expect instant friendship between the dog and visitors. Not all dogs are naturally friendly this way. Everything is like a training exercise. If the visiting kids come running in and want to start playing and touching your dog, this may be frightening to him. Teach the kids to let the dog come them on his own terms. I would also discourage kids from picking him up.