Keep it simple!
As new sewists, it's important that they get that feel of accomplishment.
How about a simple patchwork quilt of all squares?
They could choose two fabrics ... or many different ones, even a charm pack.
All according to their personal interests.
Some will go with an alternating pattern.
Some will decide on diagonals.
And some will just go all random.
You could encourage them to colour in the pattern they want on a grid.
Do it as a 4P to help them them learn about matching seams.
Then join the 4Ps together to make the quilt.
More detailed patterns can come later once they get more confident in their abilities.
For now, let them see completion and success ........ with huge smiles!