If you're worried go to a speech therapist for advice. The best thing you can do is just give time and wait for them to finish speaking. Speak about it in a neutral way and acknowledge it if you think your child is frustrated or aware of it - but best to check with your speech therapist first. Therapy at this age would likely focus on parent-child interaction. There's lots of research into persistence of stuttering and things like genetics/ family history, child's language abilities, how long they've been stuttering and their environment all play into it. It's hard to tell especially at this age which kids will stop stuttering and which will continue. Around 8% of kids stutter at some point, and around 2% of adults.