12x is plenty for airgun ranges, especially when you are starting out.
Are they shooting rested from a bench beenbag, or doing it properly ?
Also why SF ? I don't really get the requirement in your scenario, so it would be good to understand your thinking - unless you wanted 24 mag; in which case, sure, as everything will be out of focus. So don't go 24 mag, problem solved
In fact, if i wanted lightweight, I'd go fixed paralax at 35 yards, with something like a humble 3-9x40 so they can use lower mag 4x or 6x for standers and 9x for sitting/prone. ON my kid's first PCP I had a lightweight 4x28 fixed PA (set to 25 yards) scope. Worked well for them out to 25 yards.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.