Hey, Im in the same boat as you, but I think one way of doing it is making the sculpt out of some hard clay, make a plaster mould of it, and then using that plaster mould, cast the vacu-form mould out of resin or some hard material of the sort...
There are diagrams I found online that have all the pieces minus the helmet, with all the measurements, Im using that for the molds I will help make for the group Im gonna be working with soon. Fortunatly, one of them already has a vacu-form machine...
For short run stuff, or an intermediate mold I wouldn't form directly over more than a few times, I might use something like water putty or gypsum cement, with drywall tape reinforcement.
For something really durable, I'd probably go with a resin impression coat, and lay up some fiberglass of some sort under that. Maybe just a healthy layer of Bondo-Glass. (Which is not like regular Bondo. It's not easy to sand, and it's much stronger, because it's filled with glass fibers instead of little microbubbles.)
If I still wanted to be able to sand and modify it easily, at the expense of making it somewhat less durable, I might start with a layer of regular Bondo. Then, deeper than I'd ever want to sand away, a healthy layer of Bondo-Glass for overall strength.
BTW, don't trust me on this. This resin/glass stuff is all still on my to-do list, not something I've actually done. (Today I'm experimenting with hot wire cutters for the foam part... I'll get there eventually.) I have done plaster waste molds and reinforced water putty casts, but not fiberglass.
You want the best way to learn to do molds. Go to the " YouTube" web site and type in Resin Molds, Mold making, or some other variation of that. They have tons upon tons of how to videos showing you the material needed to make accurate and structural sound molds.
I dunno, I was using an electric saw (portable jigsaw) when I was 12. I don't see vacuum forming as being that dangerous. You might get a painful burn, but probably nothing disfiguring or even permanent, and you probably won't cut off any appendages. The most dangerous part is trimming the plastic with Sharp Things.
Whether it's something a 12-year-old ought to be doing depends a whole lot on the 12-year-old.
Umm you have to put plastic in your oven. I think thats a bit more dangerous than using a jigsaw. The only people I've seen get hurt with a jigsaw were really stupid. It's practiaccly impossible to hurt yourself with one unless you're trying to.