Instead of party hats every little guest is given a pair of elf or fairy wings to wear.

'Plant' big, pretend toadstools and mushrooms throughout your garden to give a sense of scale (i.e. mushrooms are big to fairies etc). You can get cheap polystyrene ones (may need to do some hunting for bigger ones) and paint them.

Make edible wands - i.e. Pop something on the end of a stick/skewer like a star shaped biscuit, make it all look pretty.

Birthday cake or treats can be served on a 'platter' consisting of a big square or rectangle of mock grass/astro turf type surface made up of mushroom or little cottage cakes, flowers etc. You can add some plastic props such as little deer, fairies etc. Basically make a miniature elves and fairies kingdom.