you can download a cheat that you can get from www.wilkaresfunsims.com
. that site has a lot of helpful *hints*.
After you downloaded the adoption agency download, download the Add-On Handler thats on this site somewhere. You can download it from this site. Don't worry these downloads are safe.
Oh ya, if you wanna do it normally, just kiss and hug your wife a few times while your at 100% relationship and a question will pop-up saying "Do you want to have a baby?"
Hope it Helps,