I would be ok with this myself as I am diligent with data use, but my son's 4s went haywire and used 5gb in one month, even with WIFI always on and connected. I would be in the poorhouse after one month!
You wouldn't be in the poorhouse because you prepay. They don't send you a bill after the fact. If your son's phone went haywire and his account had $15 on it, then you'd only be out the $15.