I would personally buy a 3m 6900 series full face respirator in a size smal. Add p100 cartridges if he's going to breathe grinder/welding dust:
http://www.amazon.com/dp/B007QY8WTY/?tag=atomicindus08-20
I have the big kids version and love it -- it's super comfortable.
If you give him a regular face shield instead, then hot metal bits can still get under it, bounce off his cheeks, and hit his eyes. If you add eye protection under the face shield, this can still happen (unless they're goggles that seal on the cheeks).
This has happened to me many times while running and angle grinder and a few times working with woodworking tools.
In contrast, a 3m or similar full face cartridge mask seals around his forehead and chin...no dust or fast moving bits can get in. It's certainly not the cheapest option, but I wouldn't look for savings on your child's PPE. Plus, the 6900 is sure to meet mom-approved protective standards.
Besides 3m, honeywell and a few other companies offer comparable full face respirator masks. You can usually grab one of them in a size small for ~70-80 on amazon prime.