Either Bo Bae or Pratunam are just fine. My wife can get similar prices at either location. However, I suggest you confine your plans to only children's clothing.
The sizes they offer over here are small, extra small, and so small that they had to do the stitches under an electron microscope. Yes, I have seen farang sized clothing in a store or 2, but as a general rule, these are Thai shops for Thai people (and the XL sizes cost more when they are available).
Unless you are doing a product line for midgets, you won't have too many opportunities. Generally, for shirts, the prices are about 200 - 300 baht for a set of 5, and 5 is usually the minimum qty. For the really cheap items you can sometimes get less than that, and there are items that will set you back significantly more, but that is a good average.
The quality is similar to what you could expect to find at your local landfill, only in this case they charge you per item of rubbish you haul out.
Yes, it really is that bad. But that is what people want. My wife only wears a shirt once and decides it is old anyway, so why pay for quality? If you need designer labels, just buy the label. They're generally available, although they tend not to be displayed. A friendly chat and a good sized order though and the shop keeper can probably arrange with a contact to have the right brand sewn on. That's alot cheaper than paying for the whole shirt. They're pretty careful about that now though, so you it might take some convincing and a good repore with the person you are asking. It's not like the old days.
There are deals to be had, but if it was easy, everyone would be doing it. Plan to spend alot of time looking around. You sound like you understand exactly what you are getting into, so the best I can offer you is my hope that you'll find something you want.
Finally, make sure you have a Thai do the initial negotiating for you. Don't even show your face. You can pop in later after the haggling gets serious and you're ready to pay. Just makes things easier that way.