I have no doubt that your niece will thoroughly enjoy both listening to and learning these delightful pieces ! There's enough variety in here to keep a beginner at the piano thoroughly engaged.
Given what @Bělásek mentioned above, if you decide that the cello double stop is too difficult you could also have the viola take the G and it would be an easier double stop for viola G and Eb.
If it is not in some crazy tempo it seems doable to me. It is not comfortable, but if You write Sul G (and maybe allow some little gliz) in the preultimate measure in cello it could work.