Deirdre,
Floating and sinking is a staple of the infant classroom and allows children to closely observe and describe what they see as they attempt to sense build around ideas related to materials and to forces.
I’d be cautious about accepting “guesses” (even though that is the language of the primary curriculum skills for infants in Science), since their prediction should be linked to their emerging science understanding. If they are merely guessing they might as well flip a coin to state if an object will sink or float, and they have no understanding that can be improved on.
I’d suggest that you let them play with a variety of objects, group them according to if they float or sink, use similarities between the floaters or the sinkers to establish a simple understanding, and then, with that understanding let them choose other objects (an ‘informed guess’) that they believe will behave in the same way – either by floating or sinking.
The glossary in Science states:
floating: the tendency of an object to remain on the surface of a liquid; an object will float if its density is less than that of the liquid; the weight of a floating body is equal to the weight of fluid displaced; the force up and the force down on the body are equal
This is the only place where density is mentioned.
In work on weight and capacity, density may be referred to, but generally children might refer to a material as being heavy for its size (a metal cube) vs light for its size (a plasticine cube).