I am still nursing my son who is a year old and I have lost over 20 lbs since January. I don't drink soda very much because of the negative health issues with soda. There are a whole host of reason to just not drink it. (I will indulge on occasion at a restaurant or something but not often.
You can lose weight by drinking small glasses off your beverage of choice and then the rest of the time drinking water. I typically have two cups of coffee in the morning (about 100 calories) and then I drink a small glass of tea at lunch, a small glass of juice in the evening and water the rest of the day.
I figured out how many calories I should eat to lose weight and I stay within that range. I keep my fat grams down below 45, I watch my portions (1 meat serving a day the size of a deck of cards, 1/2 my plate are veggies, 1/2 cup of starch, more fish and chicken, no snack foods like chips, cookies...)
I research all the time on sites like realage.com and nationalbodychallenge.com
I have several books the my MIL got me from thrift stores and I gleen information from those...like Prevention Magazines book called Your Perfect WEight. Another book called The Portion Plan, a Weight Watchers book, another called the Maker's Diet... I don't take any of them word for word but gleen what is good for me.
I also pay very close attention to healthy lifestyle habits for my heart.
You can lose weight without drinking diet drinks.
I don't believe it will harm your little one in any very apparent way, but who knows what hidden effects it might have...just avoid diet drinks as much as possible. A treat once in awhile shouldn't hurt him, but I wouldn't drink them daily.