Present the children with the scenarios and get them to respond with what they view to be potential positive, minus and interesting points/ outcomes. When questioning children about their responses or getting them to share their ideas keep an observation record at hand to note down any gems they come out with. Often great science reasoning is lost in terms of evidence as we don't capture pupil talk.
In KS1 I would conduct the PMI has a whole class exercise.This could move onto it being a group activity that is then feedback/ debated as a whole class when they move into Year 2. Therefore, when they move onto KS2 they will have had experience of the process before being expected to respond individually and share and explain their own ideas. The emphasis at this stage should be modelling how to explain their ideas and , most importantly, their reasoning behind their ideas.
Year 1 PMI
Year 2 PMI