Make the drummer play with brushes and make everyone else turn down.
No, but in all seriousness, I know how this goes. I was in a band when I was around 15 that had the same problem. Drummer too loud, no one could hear each other, everyone kept turning up their amps that were way too big for a garage, too much sound for such a small space. You know what happened? I'm 26. I have tinnitus. Mild in one ear, severe in the other (the one that was close to the drummer all day.) And let me tell you, a constant high frequency ringing in your ears that NEVER goes away is very frustrating.
I'm not sure what you should do. But do it quick, for the kids' sakes. IEM's seem like using a fire hose to put out a candle, but that's just my opinion. I would suggest seriously getting the drummer to play softer, and padding the hell out of that room to minimize sound reflection. Also, get them to wear the earplugs. Those hi-fidelity ETYs aren't so bad. You kinda have to push them in far enough in order to reach a spot where the sound cuts through again without being nothing but bass, so make sure they do that.