The question I would have to ask is, how do you define a child? In Texas, up until AFTER the FLDS moved there, a 14 year old girl could get married. Due to lots of rumors about the FLDS, the state changed the law so that a girl had to be 16.
What was not reported about them is that in every case of a young woman getting married, it was the girls that demanded it, not the men.
It depends on the culture a person is raised in as to what age they are going to act and think like an adult. While the youth in the FLDS and other, similar, groups tend to be very innocent about the world, they frequently accept and live up to responsibilities that many "adults" can't.
I don't condone forced or coerced marriages of anyone, especially not "children". Unfortunately, young girls ARE forced into marriages in some cultures, often as multiple wives.
So, what definition of a child are you using? Are you going to accept the state's (or nation's) definition? If so, then we all need to ensure that it is enforced equally, not just for polygamous marriages (which is what Texas has done).
If I am not mistaken, 18 is the nation's age of considering someone an adult in the US? I think anyone over 18 could decide whether they want to enter into a plural marriage or not. Younger than that should be illegal (in my opinion).
Each state has its own definition of what age is acceptable for marriage, as well as a legal age for sexual relations. If a person can decide on marriage at a particular age, then that person should also be able to decide on a plural marriage. In my opinion, anyway.
Most everyone you talk to around this nation will tell you that your not an adult until you reach the age of 18. Granted some states will allow a girl of 15-17 to legally marry. Even so they are still not considered an adult until they reach 18. SO I would say that even in plural marriages that it be with consenting Adults.
Why would anyone want to marry a fifteen year old? I mean you haven't even finished high school yet and probably still collect Barbie dolls. Not just that but a fifteen year old won't even know if they want to get married or even taken on polygamy at that age.
I agree that in the current culture almost all 15 year-olds are still children. However, in the late 1800's, a 17 year-old woman who wasn't married was close to becoming a spinster. There are some subcultures, even in the US, where a boy becomes a man at about 16 and a girl becomes a woman at about 15 (in the emotional, responsible sense).
I would be VERY hesitant to assume cultural "superiority" over those subcultures, considering the extremely high rate of teenage pregnancies, abortions, and STDs in our common culture that are not found in those subcultures.
Anyway, I am not a proponent of young women getting married before 18 either. I just recognize that there are some situations where a woman of 15 or 16 getting married (whether monogamous or polygamous) can be better and more stable than if the woman is over 21. Heck, I know young women over 21 who are FAR less mature than some 15 year olds I know.