The Doctor knew Clara wouldn’t let him get away with it right after Leo was born, so he waited ‘til she fell asleep and snuck with their child into the console room. He climbed the stairs to the balcony - he needed to be high up - and used his sunglasses to start the soundtrack playing through the TARDIS’ speakers. This was something he’d wanted to do since two regenerations ago, when he’d surprised himself by quoting The Circle of Life while trying to prevent an alien invasion.
As the music swelled, he raised his impossible, perfect son into the air, daring the universe to try - just TRY - to say that Leo shouldn’t exist. The Hybrid was here, and the Doctor couldn’t be more proud.
Leo had been sleeping peacefully up to the point, but maybe the music was too loud, or maybe he just didn’t like being dangled from his father’s arms over the console room floor. Whatever the cause, he cracked open an eye, assessed his situation, filled his tiny lungs to their full capacity, and let out a screech that echoed down every corridor in the TARDIS. The Doctor flinched, instinctively moving to cover his ears before remembering that his hands were full of outraged hybrid. He fumbled, working desperately to keep from dropping his squirming, screaming son the half-story to the floor. He had no way of knowing how resilient Leo was, but he did know that Clara would never forgive him if he was responsible for causing their son to regenerate before he was even a day old.
'Shhh! Hush, Leo. You’re safe. You can stop crying. Daddy’s here. I’ve got you. You’re safe.’
Leo wasn’t having it. He sucked in another breath and resumed explaining to the universe his displeasure.
'Really, you can stop crying. You don’t want to wake Mummy.’ He was contemplating using his Dad Skills to put Leo back to sleep when his blood ran cold.
'Too late. Mummy’s awake.’
He froze, eyes darting around the room to land on his tiny wife. Her arms were crossed, and she was glaring at him.
'Ah, Clara, this isn’t what it looks like.’
'Really?’ She looked slowly from where he stood at the top of the stairs, motionless except for their son, who was starting to vibrate with indignation, to the console, which was still pouring out the soundtrack of the Lion King - although the music could barely be heard over Leo’s wails. 'Because it looks like you’re using our son as a prop while you stage an amateur production of the Lion King. What have I told you about reenacting Disney movies?’
He hung his head. 'That it always results in death or dismemberment.’
'Exactly.’ She walked to the stairs and held out her arms. 'Give.’
Sighing, he handed over Leo, who immediately ceased crying and snuggled contentedly into his mother’s chest. 'Traitor,’ the Doctor groused.
Clara came up beside him, tugging him down by his hoodie so that she could kiss him. 'Don’t be jealous, daft old man. It’s not your fault my Mom Skills are superior to your Dad Skills.’ He growled, but she laughed and began tugging him to the library. 'Come on. There’s nothing wrong with watching the Lion King. Just don’t get any ideas about reenacting the wildebeest stampede.’
The Doctor brightened. 'You know, I bet Leo would love to go see real wildebeests. And lions, and baboons, and-’
'Maybe wait 'til he’s walking, yeah?’