When people ask me "what its like" to stutter i use this exercise that i think capture the feeling pretty well. Dont know if it works on children though. It have multiple "difficulty levels" but the first one usually does it. Hold both hands on your chest. Let on of them stand still and tap while the other runs up and down. Most people cant do it. Its not like they dont understand the instructions or forgets what to do, the body just isnt able to do it. Think it captures the feeling pretty well. If thats to easy, sit down and draw a line up and down with one foot while the other draws circles. Pretty hard.