Michelle,
by starting with features of planets you have a natural connection to science when it comes to designing your aliens.
Do you have a suggested video about the planets that you might use? This NASA site has images (imagined but based on our science understanding) of planets beyond our solar system:
https://exoplanets.nasa.gov/alien-worlds/exoplanet-travel-bureau/?intent=021
We have created aliens from foil for years, using a format similar to this:
https://www.instructables.com/Tinfoil-Animal/
We review the instructions together, have the children plan ‘how many leg, how many arms, how many heads…’ so that they then work out to make 1 less cut than the number of limbs.
