I'm no expert and just going off my own experience and observations so make of it what you will Someone already said don't say "slow down" or "think about what you are saying" or anything like that. The more self conscious she becomes about it the worse it'll get. It might just go away by itself but if it doesn't and it begins to affect her mental health. The best option is early professional help, and the best thing you can do if that scenario happens is make home a safe place where no one draws attention to her stutter. Where she doesn't have to worry about it or feel self conscious about it. Not all but some Stutters can be self causing and anxiety driven. If you act like it doesn't bother you, it won't bother her as much and she'll possibly stutter less because it. I'm middle aged, pretty successful and have decreased my stutter to a point where it doesn't really affect my day to day life but I still stutter the worst around my dad because he always brought attention to it and made me the most self conscious about it.