Dean will take this to his grave. Not even Sam or Cas will ever know.
Jack was cuddling in Dean's arms as Dean was reading a story. It was an oldie, his mom read it to him as a child. It was about a mother's love to her son. Once Dean was finished, Jack begged Dean to read it again. Jack told Dean, Kelly, his mom read this book to Jack before he left. He said Kelly was planning to give it to Jack on his first birthday. Unfortunately that never happened, Kelly died giving birth to him, her final words telling Jack "I love you."
Dean held Jack as he read the final words of the book.
"I love you forever
I like you always
As long as I'm living
My Angel you will be."
He heard Jack sniffling.
"Kid, you okay?" Dean saw a tear going down his face.
"My mom. I only just met her, then we were separated again."
Dean just squeezes Jack.
"You have us. We're your family too."
Jack was touched. He loved Dean, Sam and Castiel. But his mom was dead. He missed her deeply. He was trying to hide his tears, but Dean saw right through him.
"You know you'll see Kelly again. But for now, you're stuck with us. Come on. I'll make you hot chocolate, and I'll send you off to bed."
Jack knew he was in the body of a grown man, but he was just a toddler. He followed Dean.
He felt Dean tucking him in after.
"Dean?"
"Yes?" Dean was about to put out the light.
"Can you...?"
"Sure."
Jack felt his eyes closing as he heard Dean singing. Instead of a lullaby, he sang "You are my Sunshine" to Jack.
Once Jack was asleep, Dean carefully left the room. This was something only Jack and him shared. Dean did the same thing to Sam when he was about Jack's true age. Sam never remembered, but Dean did. Sam never had a nightmare every time Dean sang.
"Goodnight kid."
Jack smiled, he was safe and sound.