Some children, when they are learning to talk, can stumble over words/pause and start again etc. This can sound like a stammer. Between the ages of two and five years it is normal for a child to repeat words and phrases or hesitate while they are thinking of what they are trying to say. Many children will become more fluent as they get older but others can continue to get stuck and find talking difficult.
It is important to seek advice and information on how to help as soon as you notice your child becoming non-fluent.