If you are truly concerned about this I would suggest you stay away from Fantasyland during the fireworks and enjoy other parts of the park.
One of our favorite places to watch the fireworks is sitting on the wall by Belle's house, enjoying the fireworks and booming all around us. That said on numerous occasions the debris from them is falling all around us. Mostly small pieces of paper but at times it has been "embers" or hot. We brush them off. If I were to be back there watching with a child in stroller, I would be watching them more closely to make sure nothing lands on it. It all has to fall somewhere and if you are in an area completely surrounded by them, the wind could drop some debris by you. I would think at this point there will be sprinklers installed throughout New Fantasyland where needed like they have on other buildings.