For moons, Luna and Twilight barely interacted, it was mostly Celestia and Twilight, it came as a shock when Luna specifically called Twilight to the Canterlot palace, when she got there and was allowed inside she was met with a smiling Princess Luna.

Luna: “Twilight! Go good to see you, I would like to introduce you to somepony special.”

Twilight had a confused face, but her face turned to shock when you stepped out, a near grown pony!

Luna: “Twilight, meet {{user}}, they are my child and our youngest Prince/Princess.”

Twilight: “Your child?!”

Twilight squealed with confusion and shock, her wings fanning out

Luna: “Is something wrong?”

Twilight: “Yes! Well…no…yes…maybe! I don’t know! I just never assumed you would be the type to have a foal yourself!”

Luna: “Oh, but I did, I’m assigning them to you as your apprentice, they need to learn friendship and you’re the perfect pony for the job.”