For what it's worth, the multi-user experience in my case has been pretty seamless. Here's my setup if it helps anyone:

My roommate and I both have separate steam accounts (it sounds like you may be looking for a 'child' account or something like that, those may be a thing but I'm unfortunately completely unfamiliar with that, so ymmv if you use that).
We set up family sharing between us to access each other's games, but did that I think entirely on a computer via that steam client. No pins or anything were necessary iirc, just a slightly convoluted sequence of logging in and out of steam on the same computer and clicking the needed 'family sharing' buttons.

Then I set up the deck with my account, logged out, and had my roommate log in. There's an option somewhere to start the steam deck at the account select screen every time it turns on rather than automatically logging in to the last used account.

It sounds like most of the difficulty is coming from the family sharing setup. Like I said, I'm not knowledgeable on if steam has 'child' accounts that can be linked to other accounts, if so it's possible that none of what my process was like applies.

Hopefully that's at least somewhat helpful

You're not doing anything wrong, the multi user experience ootb is awful if you share any games.

As a single user, the steam deck has been basically perfect. This is good info though when I’m talking it up to other people.

There are a few different issues interacting here.

1. The "family mode" users that require PIN are a child protection measure, and are not connected to Family Sharing. Remove the PIN from all adult accounts. Now you will see your whole library and be able to go to the store, and when you switch to your son's user, he will not be able to go to the store and will only see the games you have done "Add to Family Games" on. This is how my library is set up: sharing to my partner and child, only child's account has PIN.

2. I don't know the cause of your experience with the keyboard, but if you remove the PIN from your own account, that should make it less painful.

3. This is just the way the Steam client works, not a Deck-specific feature: you are logged into one account until you change it. The PS5 is the same way.

4. In my experience, failure to separate game state between users is a game-by-game problem. Most Windows-native games running in Proton separate their saves by user correctly. (I do not know whether this happens because the Deck generates a completely clean Proton environment for each Steam user, or whether the Proton environment is shared and the game is just doing what it would do on a Windows PC to separate saves.) The games where I have seen saves wrongly shared, ironically, are all games with native Linux ports.

5. If you haven't already, switch to your son's account, unlock the PIN, and go through all the Steam multiplayer/chat settings. We have all that turned off for our child. As far as I know, a game family-shared to a user should behave exactly as if the user owned the game, from a functional point of view.

Dang, that's good information, and is pretty much a deal breaker for me. 😞 I've been on the fence for a while but I guess this pushes me off that fence for now.

To do multi-user correctly it will take them rearchitecting a few things. Here's what I'd image is currently required:

1. Game artifacts would need to move into a common area to prevent duplication (or utilize a COW filesystem)
2. Game compat data will need to move under the user home directory and not immediately live with the other artifacts like it does now
3. A new login screen will need to be created that actually switches between system users (will need to use some password separate from an account password for offline use)
4. Some extra state tracking may be useful. Switching back and forth between desktop and game mode will prompt the account switch menu but some data can be stored to make it remember who the current active user is...

Most of the tools to do this are already present but it'll take some time for someone to coordinate it and the fact that the product has made it this far without such a feature speaks to it's demand. Hopefully someone takes a look at it though because it really shouldn't be that bad.

The one thing I would really like them to improve on is the process of switching users. I have a Chimera system in the living room and it's so many steps to switch accounts.

Waiting for steam to "shut down" when switching users is very janky.