No, I believe you could just remove the ear cap if it is removeable and cut some of the space out between and move everything together. You might have to cut around the earpiece. Then you would have to glue, etc. everything back in place.
I'm thinking it would be a lot less work to just get one of those child size helmet kits already available; it's just interesting what's out there and, as always, contemplate what they "should" have done. Who knows, this is a prototype, I believe, so maybe the final product will be more squinted up.