This question does correctly identify the fact that no hypothesis is perfect. As a start, your initial hypothesis is a fairly decent hypothesis. It is specific in its wording. Additionally, the hypothesis is phrased in an "if/then" format. Not every hypothesis has to be worded in this format, but the format does help strengthen a hypothesis, because it helps to guide students toward writing a hypothesis that makes a prediction of some kind.
The other advantage to using the if/then format is that it helps to identify the variables. The first half of the sentence identifies the independent variable(s)—in this case, the cymbals and the bunnies. The dependent variable is the development of fear.
Looking more closely at this hypothesis does reveal weaknesses within it. The first problem is that the hypothesis contains multiple independent variables. That is something that should be avoided at all costs. The children might develop fear, but you have no way of knowing if that fear is a result of the bunnies or the cymbals or both. Additionally, you really have no way of knowing if the sight of the cymbals causes fear or the loud cymbals cause the fear. It is the same with the bunnies. Bunnies might induce fear no matter what. They might not have to be "fuzzy" bunnies.
The best thing to do to reduce the number of possibilities is to reduce the number of variables. The consequence to this is that you will have to write (and test) multiple hypotheses in order to find the possible connection between fear, cymbals, and bunnies.