If you want to play relaxing pieces for them that they've heard, I see no reason why La fille aux cheveux de Lin and the Op. 9 No. 2 nocturne won't work. Both are pretty fast to learn as well.
Besides that, I guess anything else from Children's Corner or the 9/1, 15/3, 32/1, 32/2, 55/1 nocturnes are pretty easy. If you want a faster, brief piece that sounds much harder than it is, the F minor, E-flat minor, C-sharp minor, and C Major preludes from Op. 28 fit.