How dangerous is soldering lithium ion phone battery, and how easy is messing up
Have you looked at a phone battery? Your mention of spot welding doesn't match up with what a phone battery is. Spot welding is for batteries with hard cases and direct contacts or tabs instead of wire leads. Not to mention how you plan on charging these random scrap batteries to begin with.

I wouldn't solder lithium batteries as part of a workshop.

If they are phone batteries they will either have:

Leadless, direct contacts: It's too easy for beginners to overheat and damage the battery. Even if the battery doesn't catastrophically fail, you now have a damaged battery that may not look it and end up being hazardous in operation later on.

FPC (plastic film) connectors: Basically thin flexible plastic strips covered with tiny metal traces. It is too easy for beginners to short the closely spaced pins together while soldering or melt the plastic film (which could also result in shorting the battery). In any case, if that happens someone is going to have to grab the battery, toss it in a metal bucket, and make a dash for the door to toss it outside as it flames and smokes. Even if these two things were not issues, it would be fiddly to do and not suitable for beginners since they were never meant to be soldered to. Beginners have enough trouble soldering things that are made to be soldered, let alone something that was never meant to be soldered and would therefore require 4 hands.

Get batteries with real wire leads on them so you don't have to solder directly to the battery, or better yet pick a less volatile kind of battery.  For an LED device, you do not need something as powerful (and volatile) as a lithium battery. AAAs or 9V would do fine and are standardized so you can get standardized wire connectors for them. I would prefer 9Vs for because with AAAs you would need more than one battery which means a bulky holder.

Keystone Electronics