It is understandable that you want to show him that parenthood doesn't have to be so hard. It is normal to want to have your own children to further your bond with your spouse. However, please give it time. It really is better to allow the entire family to adjust before bringing more children into the situation. If your biological clock is ticking I can understand the rush, but if you have years before you have to worry about that consider giving your marriage about three years before bringing more children into it.
Most second wives are in their first marriages.
Looking at these marriages it is easy to figure out that added children cause more stress when the first situation is not resolved. Over all most marriages that are going to fail do so within two years of the woman giving birth. There is no doubt that children, financial problems, and unresolved issues with ex spouses contribute to the high divorce rate among second marriages. It doesn't have to if you take your time. Don't obsess about the situation with his Ex Wife.