Many young children speak dysfluently at times, especially when they are under some pressure to speak. There is no exact point at which normal dysfluency becomes stammering though there are features which enable us to decide between normal non-fluency and stammering.
Normal non-fluencies are usually relaxed repetitions, often of whole words either at the beginning of a phrase or when a child is thinking of how to finish a sentence.
There is a greater risk of stammering developing when the child often gets stuck on words, prolonging or repeating part of the words or putting excess effort into finishing them. It is also a concern when the child seems aware of and upset by their dysfluencies. However, one of the things that make it so difficult to say with certainty whether or not a child stutters is that there can be so much variation from day to day and in differing situations.