Context · Games
Fortnite is more than a shooting game, it's a social hangout, a concert venue, and a marketplace, and it comes with real social pressure baked in.
Why this matters for your kid
It's a free to play game where 100 players compete to be last standing, but the shooting is almost secondary now, kids use it to hang out and voice chat and attend events and express identity through outfits called "skins", and that's what makes it stickier than a regular game.
Why your kid wants the skin
Skins cost real money via V-Bucks, the default skin marks you as a beginner at school, and so kids face real pressure to buy skins just to fit in, and that pressure is built right into the business model on purpose.
Why voice chat is the part to talk about
Whether voice chat is on depends on the age set on the account. If it was set up as an adult or skipped the age step, open voice chat with strangers is on, and your kid can hear and be heard by anyone in the lobby. Accounts that say under 18 default to off, and under-13 "Cabined" accounts stay off until a parent gives consent, so the real question is which kind of account your kid actually has.
What you might notice
You might notice V-Bucks requests, voice chat with people they do not know offline, or frustration and anger after playing. Competitive games are built to be emotionally intense, so that last one is the game working as designed rather than anything about your kid.