What class/subclass for a 5 year old who is not a strong reader

[Kvess]

-My dad is so strong he can jump really high!
-Yea? My dad can fly and shoot lasers from his eyes!

Jokes aside, the "edge" of the champion in the activities you mentioned is a character sheet thing too. Every STR character can do stuff like that. Not as good as a champion perhaps, but that's really a matter of optimization, isn't it? Also the champion can do these things better at level 7, while the warlock can make someone pass out from laughing as early as level 1.

Maybe I wasn't clear enough. It's not that the Champion Fighter doesn't have to have a character sheet, but the player doesn't have to look at it. Being a strong person with a sword can be incredibly simple and immersive, in the sense of "What do you want to do?", and you can spend more time engaged with the story than your spell list. I've had great experiences with the subclass and I'm an adult who also DMs and plays casters. A typical campaign world is full of objects just waiting to be climbed on and toppled over, and there's fun in that.

A kid could just say: "I want to smash open the door!" And the DM can say: "Alright, that's an Athletics check, roll a d20."

With a Warlock casting Hideous Laughter, the kid needs to know what a spellslot is and how to manage them, how line of sight and range works, which creatures have an intelligence score of 5 or more, which creatures are more likely to fail Wisdom Saving Throws, and what the Prone and Incapacitated Conditions mean. It requires more rules knowledge and book keeping than, "I want to wrestle that dragon!"

And I don't think smashing open doors or wrestling dragons has to be boring. Whether or not your character is boring is up to the choices you make at the table.