My neighbour is seriously into fairies (although, of course, she’d spell it faeries). Even before she had a girl child. Now it’s ramped up to unprecedented levels.
Luckily, she lives in the countryside, with a big garden. Everything must be beribboned or decorated, certainly every tree has its fairy inhabitants. But she is adamant about no plastic. She crafts things herself out of twigs, shells, pods, etc. Far more beautiful and creative than store bought tat. She also buys from Etsy- there are some makers of fairy door accessories on there that you wouldn’t believe. Wee bridges, teeny woven things, mini lanterns and postboxes. All natural, biodegradable and truly lovely!
I even made her a tiny broom from a twig, some straw and an inch of twine! She was a bit 
at the fact I used glue, but generally appreciated the effort.
If you can decorate the forest using what’s naturally there, I say go for it! It’s a great way to get kids interested in conservation, being outdoors, using imagination and a host of good things.
By the way, you can easily introduce colour by painting (dead leaves, pods, twigs, straw) with watercolours. Just steer clear of toxic pigments (principally ones with cobalt and cadmium in the name). You might have to spend a bit more than ‘craft paints’ prices for proper watercolour, but you don’t want to poison the lil critters!