Before I start, this is going to be sad. Really sad. So in my interpretation, Celestia is incapable to have kids. Luna can have kids, but the first ones she produces will be her only children. Princess Luna has Sundrop in secret after the defeat of Discord and before she turns into Nightmare Moon. Her father is a midnight black stallion with a silver mane and tail. Sundrop gets her mane and coat color from the magic flower that saves her mother and effectively her own life. She is a unicorn until she can earn her wings. with that out of the way, enjoy and no haters.
Once upon a thousand years ago, The Royal Sisters Princess Celestia and Princess Luna ruled Equestria after the fall of Discord. As the days stretched on, the sisters knew one day they would need an heir. "Sister," Luna began " how will we solve the problem of an heir? I mean, because you can not have children and all." Luna had asked. Celestia raised a hoof to her chin.
"Well, there is always your child. The one you will hold in the future any way." Celestia mused.
"Heh, yes. My offspring..." Luna trailed off, rubbing her belly. The dark blue mare had been with holding information from her older sister. Luna had fallen victim to the charming wits of a guardspony by the name of Nightbeam. In short, Luna was pregnant with the heir to the throne of Canterlot and ultimately Equstria.
"Is it not time for you to begin governing the night, sister?" Celestia asked as she lowered the sun. Luna looked up, lowering her hoof from her stomach.
"Y-yes, it is. Good night!" Luna called as Celestia retired for the night. As Luna raised the moon up, she knew that if she wanted her filly to have a choice in their life, Luna would have to hide her child from the world.