Strict Discipline
After checking through all of the scrolls Xiao Rong had completed, Su Yang nodded to himself, "Although she still thinks like a child, her knowledge on the human language would not lose to an average human adult."
Su Yang was pleased by the results of her progress in just a few days time. At least she won't have trouble speaking to anyone that is not him for now.
As for her state of mind that still resembles a child, there's nothing he can do besides allow her to grow naturally. However, since Xiao Rong is also a Phantom Cat and a beast in general, her growth in that aspect should be overwhelmingly quick.
But because she's spent her entire life isolated in the desolate Immortal Treasury that lacked everything required for a living being to grow, her state of mind was frozen without any chance to grow.
This is also one reason why Su Yang encourages Xiao Rong to go 'play'. He's only trying to allow her to grow in the wild, where all beast normally grows.