Polygamy is just several marriages(all man-woman) running concurrently. Each wife marries the man,not each other.
It's still better for children than homo "marriage",because the kids still have a real mother and father,and benefit from the invaluable experiences of that.
Also,while one wife can be watching the kids,the other wives can be doing household chores or working outside the home,so there's extra income for the entire group,and the kids are always cared for,there's always a mom around. No wife gets burned out,because they can take turns watching the kids and doing housework.