Explosives would be hard to make in the Adstring environment, especially complicated ones! With time and some chemical knowledge, black powder or guncotton might be possible to manufacture, but it would be a laborious process with risks of possibly blowing your own arm off.

Using magic to create explosions or shooting sparks would be perfectly OK, but building mundane fireworks from scratch is tricky and requires a lot of fairly well purified and processed chemicals and explosive charges.

Basically, our limit would be: does your character have the know-how (with Fred and George, clearly not a problem) and do they have the time to build it from scratch, a realistic source for their materials, and a place to refine and manufacture their explosives (this is the sticking point.)