Success! Sort of.
After grinding a lot of material out of the inside of Trey's helmet, I was able to get it on his head. The problem is, it only looks right from certain angles. If he looks up you can see his chin poking out from under the bottom. His eyes aren't in the right place either:
Since I can't get my sister to change the size of her kid, at some point in the not too distant future I'm going to have to make a slightly larger size version of this helmet. Possibly two.