Anna,
I’d change the order slightly in the activity, and give the children some experience with floating and sinking actual objects before asking them to
using critical thinking they must figure out which objects float/sink
Their own mental models of why things float and sink can be built up through play and observing a range of toys or other items in the classroom.
ESERO have a suggested range of objects (the ones that are dropped in this module) that can also be floated (see https://esero.ie/wp-content/uploads/2015/01/04_Does-Saturn-float.pdf). In this case, they can explore, draw generalisations, then make predictions for other objects to float and sink.
I’d actually disagree with the statement at the end of the Saturn sheet –
It is important that the children discover that whether an object sinks or floats depends on the material it is made from and not its shape; it is not dependent on the size of the object.
Since you can shape plasticine to float by making it into a boat or raft shape.