intelekt22 - Is this the pack you're talking about - https://www.technicpack.net/modpack/the-1122-pack.1406454
Same type of experience, just bought a java version for my other kiddo. Who can't play on the server running at the house though the Technic Launcher. Until Microsoft releases a method for 3rd party launchers to authenticate accounts... or revamp their methodology as to when an active session is created we may be out of luck.
Experimenting with the pack to see if I can get the workaround functional.
Note: Doesn't matter which account on technic logs into to grab the modpack. There may be some difficulty in dealing with locating the appdata folder for a particular user if you are moving between windows profiles. If that is the case let me know and I'll see about explaining it. To shortcut that entire process just run the technic launcher on the windows profile your kiddo is going to be playing on. Log in as you to get the modpack and then log out. You basically just want the 'curated' list of mods in a single convient location.
So far it looks decent. Steps I've run through.
1. In the Native Minecraft Launcher (that has you login with the microsoft creds or mojang creds) run the game at least once. You've likely done this several times, so this is probably done.
2. Go download forge for minecraft 1.12.2. Point the installer for the client at the minecraft folder. If you used default settings you won't need to change the setting. When you run it, it will install a profile for forge.
3. Run the minecraft launcher again, launch forge 1.12.2. Update the installation to run with more than 2G of ram. This is the option that might read "-Xmx2G ...." This reflects 2 GBs of ram as an upper limit that is reserved. If you pick too much ram you may find periodic lag for garbage collection. If you pick a number that is too small you may find a stack overflow (stack meeting the heap). 4GB is just about the minimum that you MUST give in order to even attempt to launch this modpack. Watching the memory utilization was rather entertaining as it was building... I would suggest 5GB. There were sufficient hangs in the start up processes waiting on garbage collection during that time at 4GB of Ram. This will create the mod folder you need and do some other behind the scenes stuff. Close it down after it launches.
4. In one window open the %appdata% folder and .minecraft folder and find the mods folder.
5. In another window open the %appdata% folder and .technic folder and open the mods folder for the version you are looking to transplant.
6. Select all, copy the mods (folder's included in the mod folder) into the .minecraft mods folder.
7. Launch the native launcher and try to run the now modded forge profile.
The game at least launches at this point from the native launcher. I can't speak to if it will match the mod list on the server. Please give some feedback if this worked for you.
Notes: I've gotten one crash report of mantle and tinkers construct causing an issue... though it was with a stack overflow because I didn't allocate enough ram (forgot to alter the java arg initially) It didn't show again as I launched at 4GB. There are fairly substantial lag times with 4GB of ram.
Additionally discovered - [Edit]
The server I run at the house launched from forge 1.12.2 wont accept clients from technic. Something about a missing registry entry. I haven't solved it... though if you run the same process and launch everyone from the native launcher the connect fine. I have no idea what the exact difference is... sorry if this is jacked up. I hope microsoft provides some kind of rest API or something for the 3rd party launchers to be able to use.