I think you could give her some homework that is a little more challenging.
Could you give her something slightly more difficult?
ーI think you could give her some homework that is a little more challenging.
「もう少し難しい宿題を出してもらってもいいと思います。」
challenging で「困難だがやりがいがある」という意味なので、a little more challenging で「もう少しやりがいがある・もう少し難しい」と言えます。
ーCould you give her something slightly more difficult?
「もう少し難しいものを出してもらえますか?」
slightly more difficult「もう少し難しい」
ご参考まで!