Print out previous scientific experiments. Have your students identify parts of a hypothesis, which are correlations, brief analysis and variable identification, and the similarities between all of the hypotheses.
Emphasize that each hypothesis must be testable and that it can be proven wrong. An example of a poorly written hypothesis is "Humans breathe because they need oxygen." Yes, humans do breathe, and yes, they do need oxygen. However, there is nothing to test in this hypothesis, and it is a fact.
Find a new experiment for each child to perform. Have each student go through the scientific method and identify the question, the method and the possible conclusions. After each student has brainstormed about the experiment, have the students construct a hypothesis. Each hypothesis must show critical analysis of the possible conclusions.
Ask each student to present their hypothesis. If the student is shy, you can offer the possibility of handing in each assignment. Regardless of the method of presentation, offer constructive criticism for each hypothesis.
Repeat the steps until the students have a good grasp on hypothesis construction. If there are any students who are struggling, offer your personal assistance or a tutor.