That sounds so cool! 
When it comes to fairy gardens, I’d say it’s fine to use whatever you have around to make your fairy garden - it’s the same as deity offerings, there’s a lot of value in the things you craft and design yourself. I’d suggest using natural things whenever you can, natural structures and decor tend to be appreciated by those who live in tandem with nature.
There have been some great fairy gardens shared by coven members over the years, so if you’re looking for inspiration, here are a few discussions you might take a look at!
Good luck and I hope you and your son have fun crafting your fairy garden! Please share pics (if you’re comfortable doing so) - I’d love to see how it turns out! 
Blessed be