From having a stammer my whole life, I can tell you the best thing you can do to help a child with a stammer is to get them used to talking about their emotions and how they feel about their stammer. It’ll help them later in life. Another thing you can do is instead trying to ‘fix’ it you can tell them that it’s okay that their different from everyone else, and it’s okay if they stammer. It’ll boost their confidence from a young age, playing a major part later in life.