How do I change characters in Fortnite on my very first level?

You need skins to change your character. You can’t choose between any of the default ones. One will be randomly assigned to you. Check this guide.

Your kid playing Fortnite is not inherently bad. You shouldn’t “get your kid to stop playing Fortnite”. There are many more harmful ways they could be spending their time, such as doing drugs, breaking laws, etc. Fortnite does become harmful when it interferes with other parts of your kid’s life, such as social, schooling, etc. I completely agree that it’s harmful for your kid if playing Fortnite is the only thing they do.