My advice would be to get them to see a speech therapist who specialises in stuttering. The earlier the better! I wish my parents did that for me, but back in the 70s stuttering wasn’t very well understood and there weren’t many “specialists” around. Get them into speech therapy ASAP and encourage them to use the techniques every day. Sit in on the sessions to understand the fluency shaping techniques and practice with them every day. Good luck! 👍🏼