Salam
In my opinion, the comparison of having another child is in NO WAY the same as having a second (or more) wife.
Let's look at these scenarios:
1. A man and woman are married and have a child. Both the parents are available to each other and the child; pretty much all the time (say minus working hours). They have another child. Both the parents are available to each other, the first child, and the second child, and the first child now has another loved one in the family. They will celebrate the holidays together, spend family time together, no matter how many kids they have.
2. A man and woman are married and have a child. Both the parents are available to each other and the child, pretty much all the time (say minus working hours). The man takes a second wife. Now the first wife only has him for her and her child half of his time. Which family does he break fast with during Ramadan? Who does he celebrate Eid with? What if he has children with 1 wife and not the other - does he spend more time with the kids/wife (who have each other) or the childless wife (who has no one else) - but wait, shouldn't he be spending more time with his kids - then what about the childless wife, she has to be alone more? If he has children from both wives, how will he divide his time?
I am NOT SAYING that what God has approved should be thrown aside - I am simply say that everyone needs to look at the ENTIRE situation they will be in if they choose (or agree to) this lifestyle choice. They need to have a PLAN as to how to deal with the situations like what I mentioned above.
I have personally known (in my husband's family) several of the older men who have 2 wives and not a single one of the women has ever been happy with the situation - the kids are not happy, and it makes it rough for the whole family. I am not saying that people can't make it work, but think really, REALLY hard. Many people say it is hard enough to make marriage between 2 people work these days, imagine adding another into the mix?
Just my opinions...
Lana