Finally! I found one that works, and surprisingly isn't all that sad! There will be another, likely darker one about Severus, but for now…

Dear Severus,

"Dad?"

"Yeah, Al?"

"Can I ask you something?"

Harry chuckled "Well, I believe you just did, but you can ask something else if you like."

"Why is my middle name Severus? James says it's cause I'm ugly."

Harry groaned. James was always stirring up trouble. "First off, your brother will not enjoy the consequences of insulting both you and Professor Snape. Second, you are not ugly."

"Well why did you name me after him? You named James after Grandpa and Uncle Sirius, and Lily after Grandma and Aunt Luna."

"Well, Al, Severus Snape was a very, very brave man. "

"Why was he a Slytherin and not a Gryffindor, then?"

"Professor Dumbledore once said we sort too early, and I didn't know Professor Snape when he was in Hogwarts, so I don't really know the answer to that. Anyway, when he was very young, his very best friend was your grandma Lily. When they grew up he fell in love with your grandma, but they had a fight and he called her a very mean name. Professor Snape also hated your grandpa James. It hurt him a lot when your grandma married him, and it nearly killed him when Voldemort killed her."

"Really?"

Harry nodded. "Professor Snape was capable of great love, you see. So much so that he turned on Voldemort when he decided to go after your grandparents. He spent the rest of his life secretly working against Voldemort at great personal risk. And even though I looked just like grandpa James, who he hated, he did what he could to help me. He was one of the bravest men I ever knew. You remember that any time someone tells you your name is weird."

"Wow," Albus said in awe. "That's so cool! " Albus said, jumping up and dashing from the room.

"Well played." Ginny said from behind him.

"You think?"

"Absolutely. You made sure to leave in the major facts, not shine too bright a light on Snape, and I'm sure Al is upstairs telling James all about why his name is the coolest name ever." She said, planting a kiss on his cheek.

Dad just told me where my middle name came from. As cool as Professor Dumbledore was, I think you were cooler.

Sincerely,
Albus Potter