You may want to try looking for a Croix Rouge clothing store or a Secours Populaire. I don't live in Paris, so I don't know where you'd find one, but that would be a decent start. They're sort of like Goodwill or Salvation Army stores would be in the US, as far as I understand. I know my sister-in-law living in Lyon got some really great kids' clothes for cheap that way recently.
Looking in the online yellow pages in Paris shows a few entries for Secours Populaire.