Edel,
thanks for sharing how this has worked for infants. The discussion around why they think the ice will melt faster in one place than another is the vital part of the inquiry.
The plastic containers are essential – I have seen this done with plastic bags, and the heat from children’s hands as they check the level of melting affects the speed of melting. Ice cubes that you would have expected to melt last because they were insulated were checked more frequently, so melted first!