Children shouldn't be on any kind of dating sites anyway because there are too many legal issues involved with juveniles and having sex. Polyamory sites are just fine in general--people can date as many people as they want to simultaneously. However, parents should always monitor what kids see and do on the Internet in an age of cyberstalkers.
The nature of any dating website is to form a relationship with someone, or in this case, potentially more than one. There is no way to know what a child may be exposed to through these sites, from overly sexual young people, to sex addicts and pedofiles. There is no doubt that polyamory websites should be considered adult content. Children should not be allowed to interact in those settings.
Polyamory dating sites should be considered adult content for Internet parental controls. Polyamorous relationships are immoral and children do not need to be exposed to this kind of relationship. Parents should protect their kids from off-key sites like this. The last thing a two-parent household wants is for their kid to think polyamory is normal.
Yes, polyamory dating sites should be considered adult content for Internet parental controls, as should all adult dating sites. Children do not need to know about all of the many different options for adult sexual behavior. The vast majority of parents would want their children shielded from that content on the Internet.
When considering parental controls I believe all dating sites should be considered adult content, regardless of rather or not the site includes polyamory. Dating sites are not intended for youth and most of them have TOS's that indicated participants must be 18 years or older. For this reason, it is better to block than allow.