Hi Catherine,
Design a space suit is a great example of a simple design and make for young children. Do you use the template from the ESERO activity: https://esero.ie/wp-content/uploads/2015/01/20_A-real-spacesuit.pdf
This offers the suggestion that the suit
the suit must be able to stop the astronaut losing heat;
so children might explore various materials to see which keeps something warm longest. A slightly safer activity (no hot water needed) is to see which material can keep an ice cube cold longest – and this really connects the material to how heat moves through it.