The exercise recommends that you write down a list of six different children's toys that you'd like to invent, or that you think would sell well. An "idea stopper" is a negative point about any of the ideas you've made on that list. For example, if #1 on your toy list was "Rubik's Cube", an idea stopper for that might be "too complicated" or "not enough action involved, people will find it boring." You are supposed to write down any negatives or "idea stoppers" next to each toy on your list. By seeing the negative points in print, you will be able to analyze and perhaps rebut them.
An idea for a bad children's toy would be something like Johnny Human Torch, a doll that comes with a bag of oily rags and a cigarette lighter. It is inappropriate for your target audience.
Student or Learner