Too much iron is toxic. It can accumulate in the liver and become lethal. That's some multi vitamin/mineral formulas are made without iron. You shouldn't take iron unless you know for sure you need it, the same applies to chickens/other animals.
I'm surprised you can't get Poly-Vi-Sol w/o iron. I wouldn't give a child an iron supplement unless they needed it. If you ask at a pharmacy, maybe they can order it for you. Most pharmacies don't charge extra for doing that.
My feed dealer carries powdered vitamins for poultry and livestock, they aren't very expensive, I just use those. Is there some advantage to the Poly-Vi-Sol?