My boy considers Mono to be his "best bud." He is his confident and best friend. We are in the process of making sure that if something were to happen to us that it would be Mono who raises him. My parents will hit the roof when they find out as it was previously going to be my family that would take him, but we all think that he should be raised by the one that understands him best. And that is Mono.
I certainly think kids can gain tremendously from solid poly relationships and ones that are fleeting. Poly is essentially about being responsible to each other, communicating, being honest about what we need and staying open to everything that comes into ones life. These are life lessons that I think all kids can benefit from, Poly or not. I have not necessarily experienced this in any other relationship style, but it is the foundation of poly as I understand it.
There are other links on children somewhere... where is that link now, we have one somewhere... I'm so not good at the search thing... sorry
maybe someone else will have better luck.