If your child has started to stutter then it is recommended that you contact a speech-language pathologist to seek advice for your particular child. In general terms, the onset of stuttering can be distressing for some children and for parents, particularly if the onset was sudden and unexpected as is often the case.
It is important that you reassure your child and be patient and wait for them to finish what they are saying. If they become upset or annoyed about the stutter and you can help them, then that is OK if they are accepting of this help. Unlike adults, children are still learning to talk and they are often accepting of parental help in those situations.