@borna_lk Its very unfortunate, however:
1) Bans cannot be handled on the forums. You'll have to wait for EA Support to respond or try contacting them again.
2) You are responsible for your account, (including being responsible for what anyone else does on your account, whom you had allowed access to play). Since you had allowed your nephew to play on your account and they had gotten the account banned, the chances that the account would be unbanned is low. Ideally, you should be the only one to play on your account, no one else.
I will say that I wish that players would get muted first before being banned, (unless they were very excessive), and that the voice chat could be reported in-game as well. I also think that the game should send a log of what was said between every player involved when a chat report is submitted, so there would be context. This would also help stop cases where players bait and flame others into responding, to where if they go report another player's chat, EA/Respawn would be able to see that they were trolling the player they had reported and EA/Respawn can mute or ban that player instead.
I'm not sure if they'll be able to help, but you could also try contacting one of the EA Community Managers in a direct message. I recommend talking to @EA_David, (but do understand that they tend to be very busy and may not respond right away).
Another thing, if you do get unbanned, go into the game's settings and on the first tab, scroll down until you see "communications filter", and set it to "friends only" or "no one". If you trust your friends, set it to "friends only", otherwise set it to "no one". This will prevent incoming communications through text and voice from any non-friend, (or everyone, depending on what you choose), and also know that this setting doesn't stop your text or voice chat from sending.