Ahhhh, shopping in Paris. There are so many lovely boutiques for children. Many, however, are extremely expensive and downright impractical (but....nonetheless beautiful!).
Here's my list of favorite tried and true shops for excellent quality, style and price:
Petit Bateau The merchandise is cheaper and selection better than US.
DPAM Super cute clothes. I just love looking at their website!
H and M Yes, I know this is a Swedish chain with stores in US, but TRUST ME, they have a better selection in Europe, especially Paris. My favorite in the one at 54 Bd. Haussmann
Monoprix This is like the Woolworth's of Paris. The quality is great and you will find all kinds of fun things there. In most Monoprixs a grocery is located on the basement level. I ALWAYS recommend to friends that they find the closest Monoprix to their lodging before leaving for Paris. Also, make a note of their hours of operation, which vary by store location and size.
BEWARE:
My daughter said the cutest (or maybe alarming?) thing the other day. Since she wears uniforms to school (yeah!) we don't have to buy everyday play clothes very often. The last few trips to Paris we've really stocked up on clothes for her. At dinner we were discussing our next trip and she said, "I really need to go to Paris to work on my wardrobe." She's 8!