I have a Goddaughter, but I haven't put money away for her just yet. She is 1. I also have a niece and nephew and no kids of my own. What I decided to do is buy one share of stock for each year of their birth, until I can't afford it anymore. So far, my niece has 3 shares of Coca-Cola, and my nephew just got his first share of Clorox today.
Since you are still getting out of debt, I really think that 100/month is A LOT to spend on a little baby and to take care of someone else's financial house when yours is not in complete order. I agree with Sonja in that other people may ask you to be their godparents, and while you don't have to do the same, it's just a lot of money. Maybe you can cheap out for now while you get out of debt, and then in a few years, you can start adding substantially to his account.