This question is kind of unanswerable, as pieces can only get so easy (whereas pieces get infinitely hard).
I don't know exactly the skill level of your student, but Traumerie is a good piece to experiment vocalisation with- also his Album for the Young.
Mozart is a pretty consistent composer of some fairly playable pieces.
Traumerei--along with the rest of kinderszenen is easy technically, but it's a hard piece as far as interpretation is concerned.
Mozart it's better, but still Richter said that he rarely played Mozart and didn't like him because he was just too hard.