As an uncle of many and having a good handful of step children over the years most children will either ask why I talk like that or maybe ask if Im really forgetful and I just casually will say I was born with a stutter and it makes me talk like that sometimes. Kids can generally tell if you are nice or not and I always say it just matter of fact voice or tone or sometimes use a play voice and joke with them. In my experience younger kids are generally(usually) a joy and I just inform them and move on if they are satisfied. And a good reaction can in the future them being more open minded or whatever to future stutterers they may meet