This is one of the troubling consequences of the law, which hopefully someone will raise with Rowling at some point (or possibly the films themselves will raise).
Most probably the solution is that they are supposed to keep their powers secret from their parents, with Obliviation used as a tool in this.
In reality, I suspect an attitude of "Don't Ask, Don't Tell" applies, where unless it becomes likely that their parents will expose the community, no-one in the Ministry digs that deeply into how hard that particular separation is maintained.
Most of the other solutions are somewhat horrifyinghorrifying, although given MECUSA can apparently execute members of the community without due process, I wouldn't be suprised to find out they're used.