Niamh,
what a lovely way to include aliens into design and make for Science and for Arts.
You can connect to properties of materials, using some of the language that is described in this curious minds resource:
To extend the science connection you might ask the children to sort the aliens into families, and explain why they made those choices in terms of the conditions that the aliens might live in.
So aliens with 7 eyes might live on a dark planet, so need more eyes to see!