Well, my son is 4 so I have been to a few birthday parties lately. My son always gets a little goodie bag and one of those whistle thingies you blow on. The families usually decorate with balloons. Recently the Toulouse en Famille website started providing a birthday idea every Monday morning, and it has included things like a children's farm, a museum, a bouncy castle place. I think people won't spend as much money on this stuff as they do in the US, but it is catching on.
Last year my son asked for a pirate party and I went to Fnac Eveil et Jeux and found decorations, plates, party favors, etc with that theme. They even have pinatas.
Here are a few observations to add to that:
I would say the parties are less extravagant, and French mothers are less likely to stay up all night decorating a Thomas the Tank Engine Cake.
When you throw a birthday party for a child as young as 3, the French mothers will all leave their children with you and go shopping. And despite what recent publications may lead one to believe, French children do throw food.
I think that for it to work you need something a little different - maybe hard-to-find themes. At Carrefour I've found lots of Disney stuff, and the more high-brow toy stores tend to have pirates and princesses but that's about it. It's hard to find Thomas the Tank Engine, Peppa Pig, Charlie and Lola, and other British stuff. Disney is everywhere.
There are a lot of Anglo-American families in France, so you would probably have a market from just them if it's online. I can say that there is currently a high demand for Cadbury creme eggs in the Southwest...and at Christmas everyone was looking for Christmas crackers. So I would maybe consider widening your idea to include all festive things - Halloween costumes and decorations (this is catching on in France but I find much better costumes in the US), cranberries in November, Christmas crackers and all the other things British people get excited about at Christmas, Valentines, etc.
Debra