That's the way it must be done under our legal system. There are no group crimes, just as their no group rights. Individuals commit crimes and have rights. So you have to go after the guilty individuals, in a court of law.
The innocent, well last I looked they have rights too, and the cannot be punished just because they might sympathize with the (presumably) guilty.
Of course the "child" part of all this is probably a result of the violation of the first amendment rights of the early Mormons, thus causing the breakaway sects that then devolved into these "forced marriage" and "child bride" practices.
I don't like polygamy, and I don't think it produces a healthy society. But it probably should not be illegal, as long as it only involved consenting adults. Marriage is, or was, as much a religious matter as a legal one. It's not been that long since it was strictly a religious matter, at least as getting married. States did not even record marriages during the early years of the Republic, let alone require a "permit" or license.
“I don’t like polygamy, and I don’t think it produces a healthy society. But it probably should not be illegal, as long as it only involved consenting adults.”
My attention span is long enough that I can stay focused on the child rape right now.