All characters, outfits, etc are per account. If you want them to share they just need to use the same account. I get that this doesn't work sometimes and in that scenario where they want to play together, you'd need additional accounts and consoles. We have four accounts in our house with four consoles. Also grab and unlock items in each Battle Pass as it's the cheapest method of grabbing different outfits. For us that means four Battle Pass subscriptions. If your kids can resist buying additional skins by completing a Battle Pass you'll get enough V-bucks to get the next Battle Pass for free over and over again.