Is it possible to protect saved games on Nintendo Switch from being played by other users?
Yes but this method would require having a Nintendo Online account.
First, make sure your current save is backed up to the cloud. Once that is done, disable cloud saves. At the end of the night, redownload the cloud save, confirming that you want to overwrite the existing save and then re-enable cloud saves again once complete.
I wouldn't call this method secure but it would allow you to maintain your save while giving people the chance to experience the game at whatever state you have it currently in.
When you say "It doesn't have to be secure..." just how un-secure are you willing to tolerate?
On my Switch, I have my profile attached to my Nintendo account with Nintendo Online. I then created two additional profiles for my kids. Starting a game will now prompt you to pick which user to play under and when they play, they play under their respective profiles and have their own saves under those profiles, while having access to my games. So, if you're willing to have a barrier as simple as picking a user whenever you launch a game, then you're golden.
However, there is a drawback. Since only my account is tied to an online Nintendo account and only my account has a Nintendo Online subscription, only my account can connect to online servers. While this is fine for games like BotW, Mario Kart, and Pokemon, this won't work too well for other online games. To circumvent this, you'll need to attach a Nintendo account to each of the profiles you create, and then upgrade your Nintendo Online subscription to a family plan. At that point, every profile should then have the same gameplay experience for every game.
I do not own a Switch and I am not an expert but after some internet digging it seems that it's possible to sign up for another Nintendo account and use that account when your guests want to play games.
You might even be able to simply sign out but I don't know if that will prevent people from saving their progress.
My answer is based on:
Yes, if you mean the profiles on the top left they will get their separate save files no matter what account bought the game. So let’s say you have one account connected to the e-shop. You can buy e-shop games from that account and all users will be able to play the game. For cartridge games it’s the same, all users will get their own save file.
https://www.quora.com/Can-two-Nintendo-Switch-users-keep-their-own-saves-on-one-saved-game