I assume you're talking about the part that attaches to the head, not the part that goes around the ear.
http://www.earsurgery.org/wp-content...ant-image.jpeg
For a kid's helmet, traveling at kid speeds in the driveway, You might carve a notch out of the inside the inside of the helmet so the implant sits in a little hole in the helmet. Doesn't have to be a full hole all the way through the helmet shell, just deep enough to let the foam of the helmet sit around the implant. That would weaken the helmet, but it's got to be better that riding without one.
And I'd only do that for a kid's helmet where the big danger is falling off at 8mph and needing something to stop their skull from smacking pavement. I wouldn't suggest that for an adult traveling on the road where there could be sliding and smacking secondary objects like a curb or car hood or anything like that.
Disclaimer....I'm not a helmet safety engineer. I'm making an assumption that a weakened helmet is better than no helmet for a situation where a kid is likely to fall off at relatively slow speeds and bonk his head on the pavement. Any helmet seems better than no helmet there. Especially if it's a larger helmet like kids tend to wear anyway that are thicker than an adult helmet relative to the size of the kid's smaller head.
As the kids get bigger your helmet options expand dramatically and you might be able to find one that just fits naturally. As you know, adult helmets can be found fairly small that sit much further up on the head, where kids helmet tend to drop down lower. You might also find one where that part of the helmet isn't a pressure point on the skull.