In general that is a bad idea. But we are all individuals. Some people stutter so badly that they want people to finish sentences for them. But in general it is better to let them talk it out and try not to get freaked out about it. And a lot of the treatments don't work.
People say take a deep breath.
I tried talking with pebbles in my mouth. A psychiatrist practiced hypnotism claimed he could cure me. And as children, you need to stress too that the person who stutters is not stupid and they are not retarded. No we are not. That is someting that children need to know.