LOL I was like, I know about this, I can help! I play on Realms with my Nephews (but we're both on Bedrock) and then I read @whileloop@lemmy.world's comment and yeah, you should go with their advice 🤣

The older Nephew and I prefer survival but my younger Nephew only plays creative so the kinda wonky solution we came up with is we'd start up a survival game and every now and then I'd say OK, we're going into creative mode and I'd switch over and let the younger nephew stock up on stacks upon stacks of their favourite blocks, spawn eggs, whatever they wanted then we'd switch back to survival. The two of us would go back to laboriously branch mining for precious diamonds and building a modest base out of cobble and oak planks while he would be building a mansion out of diamond blocks and sea-lanterns 😀

Thanks everyone for the educational responses! My daughter and I are now happily playing in one of her bedrock worlds on LAN using the steam deck. Steam deck setup for the bedrock launcher was a bit tricky but now it runs awesomely, in game mode, and even has the exact same controller setup. If anyone is curious about how to set this up I'd be glad to help, I had lots of tricky missteps on the way, but I got there.

tl;dr at the bottom.

You can't cross-play between Bedrock and Java. (Well...technically you can if you run a Java server and install the Geyser plugin onto it, but I get the impression you want a simple solution.)

Do we each need our own Minecraft account?

Yes. Two people cannot use the same Minecraft account in multiplayer at the same time.

Do we need to play on Realms or something like that?

Realms is basically a way to pay Mojang to run a server for you, rather than doing it yourself. Unlike a LAN game, or joining a friend's world in Bedrock, Realms and servers remain available to join even if the host player isn't playing.

sort of old school LAN style?

Both Java and Bedrock have ways to set this up. In Java, you can open a world up to LAN play. [https://minecraft.fandom.com/wiki/Tutorials/Setting_up_a_LAN_world](Tutorial for LAN play)

Both Java and Bedrock allow for multiplayer games over the internet. In Java, you can either use a Realm or set up a server on your own [https://minecraft.fandom.com/wiki/Tutorials/Setting_up_a_server](Tutorial for setting up a vanilla server). In Bedrock, you can join your Xbox friend's worlds so long as they are playing at the same time, and you can also join Realms or servers just like Java. (Sorry, I didn't see a tutorial on the Wiki for this)

One last thing, I'm a server owner who's been working with this stuff for the better part of a decade. If you want to chat more, I'm happy to help you.

tl;dr: Buy a second copy of Java edition and use that on the PC. Then you can host a LAN world from either the PC or Steam Deck. [https://minecraft.fandom.com/wiki/Tutorials/Setting_up_a_LAN_world](Tutorial for LAN play)

There is a way to play bedrock on Linux, you can install in on the steam deck in desktop mode using flatpak and the Discover centre, you just need to own bedrock on android