How do you pick what things to put and not put in your college essay?

It depends on the prompt you're writing about. Some college essays (like those for the UC system) only want information about what you did in high school - not middle school or elementary school. Like what @spicy_doodle said, if something occurred during middle or elementary school that lead to something you did in high school, feel free to mention it, but don't make it the whole point of the essay. For college essays that don't require high school only activities, only include things that would bring meaning to your essay and don't distract from the point. Don't put something in solely because you want to or just so you can say you did it - make sure the activities/events add meaning. Also, make sure you write about - what happened to you/activities or events you did/anything else - is something your passionate about (i.e. not something you did just to get the title). If it's something you're passionate about, your passion and genuine feelings about it will be conveyed through your writing.

I think it would be fine to include those events as part of responding to the prompt, but I would be careful about defining yourself by those events. Make sure you answer every aspect of the prompt, and try to weave in aspects of your personality or interests. If those events connect to your personality (for example, the death of your dog showing how you respond to challenges and grow from them), but if not, I'd leave them out. I'd also be careful with the spelling bee if it's not particularly recent, unless it impacted you in some greater way like inspiring you to join a book club or something. Hope this helped!

If it can be wiggled into your essay as a means of responding to the prompt, then I don't see why not. Maybe the death of your dog or a disease you were diagnosed with can be weaved into answering the "coming-of-age" or a "period of growth stemming from a life event" prompt. I have a spelling bee victory from the 5th grade, and it was a catalyst for my love of writing and the English language which is something I am considering majoring in (Liberal Arts/Humanities). As long as you can contort it to answer the essay prompt with enough information to make it a strong topic you can write about, anything can work.

So in a scholarship application, I had one main essay with 4 SAQs. I essentially used my entire writing prompt section to expand on my extracurriculars while selling myself and talking about what I will expand my involvement.