"Incidentally... out of interest (not confrontation) - do you believe that it is wrong, or too difficult, to have children in a poly environment?"
Your tagline is basically my situation, except I'm 28 and have been in my vee for 3 years.
My gf's baby just turned 1. I KNOW that having me in their lives has helped with their needs for babysitting, and I believe that 1) having me in his life as a loving, interested, engaged adult enhances the baby's life, and 2) having me in their lives helps, to some small degree in their struggle to maintain a functional, positive marriage during this trying first year with the baby.
So, I think it's easier, not harder.
Of course, you didn't qualify... perhaps you mean socially, not logistically? In our case, we're lucky enough to have friends who are just as out-there as we are, and family that is accepting if not necessarily whole-heartedly supportive.
As for the potential for future bullying, it's true, kids DO bully other kids for any reason at all... if it's not his parent's funny relationship, it could be his hair color or the way he dresses. At least if he gets bullied and I'm around, he'll have a third person to cry to. I mean, should gay people not raise kids because their kids might get bullied for it? Well, a study showed that the kids of lesbian couples are just as well-adjusted as the kids of straight couples, so I don't think there's much of an argument there.
Rather than trying to make our families more "normal", I think we need to try to make the world more accepting. If that means standing up to bullies (including older family members), I think that's a great things for kids to see and understand.