I'd go with different system acounts. That way their savegames are guaranteed to stay separate.

That's because on PC most games just care about the system user when determining the savegame folder, and don't care about steam accounts.

So, what I'd do is to:

• Give each their own system account
• Set up Gamescope as a session: https://wiki.archlinux.org/title/Steam#Big_Picture_Mode_from_a_display_manager
• Configure the Display Manager to use that session for their users (In GDM, for instance, it's enough to manually select it once on login - GDM remembers the last-used session per user)
• Profit

If you want each kid to have their own desktop experience, then you'd want to give each one their own system login.

If you only care about them each having different Steam accounts, they don't need different logins. You can add multiple accounts to Steam.

My kids have one Steam account each. That's all there is to it. Super simple, just like a console.