I sometimes let my kid use my phone, but I don't want them accidentally opening certain apps where they could cause damage. How do I set certain apps to require an unlock?

I've seen that some apps have this feature built in, but I'd like a way to do it for any given app.

It's a Samsung phone if that makes a difference.

FOSS is preferred, but any offline solution works.

you can create another 'user account' on android. it doesn't work quite like on traditional desktop OSes but it could do the job.

Not really the specific answer to OP's question... But I think this is a good strategy. I mean since you'd probably be handing over the phone on a regular basis... It's probably worth the effort to set up a new account and make some specific apps available there. Also stops them from accidentally calling your colleagues from work and all these things.

Maybe using a private space?

You should have an option in your security options to hide apps.

How it works can depend on the kind of phone you have, but most modern android phones should have the option.

Samsung phones have a built in feature called Secure Folder. It allows you to have another set of apps with a different profile. You can have a different password to for everything in Secure Folder so your kids can't access it.

You can use something like LockLock it's available on F-Droid. You can set whatever password you'd like.

Since it's a Samsung you can just move the apps to Secure Folder

That seems to make a second instance of each app

Thanks. Samsung doesn't allow multiple users on their phones (apparently they allow it on tablets). However, this led me to the Samsung kids mode, which turns out to be a good solution in my particular case.

Yes. But you can uninstall the "regular" one.

I know this is a bit late, but this question has been in my head since it was posted.

I knew there was a solution out there.

This does involve the use of another app called shuzuku which allows adb/privileged permissions.
It's a very simple setup.

Anyway, this, i believe, is EXACTLY what you asked:

https://github.com/aload0/AppLock

Get it here:
https://f-droid.org/packages/dev.pranav.applock/

since the original maintainer of shizuku (rikka) has stopped updating, the best fork (with many improvements and features) can be found here:

https://github.com/thedjchi/Shizuku

The current apk is here:
https://github.com/thedjchi/Shizuku/releases/tag/v13.7.0-thedjchi

Actually very helpful. Thank you!