a tiara with LEDs and transmitter that communicates with “fairies” in the trees.  The fairies are RGB leds with a wireless transmitter that communicates with the tiara and other fairies.  

Could use RF transmitters or BLE4.  BLE would allow parents to activate the fairires via their smartphone.

The fairies would light up when the tiara gets close.  They could flash in unison or flash in a sequence to make it seem like they are communicating with each other.  LED strips could be hung in trees to make it look like they are flying around.

Parents could use their phones to change fairy colors, light patterns, and behavior.  

Fairies could have microphones to react to sound.  Maybe they only light up if the kids are quiet.  Get too lound and they turn off and then back on after a few seconds.

Fairies should be water proof, easily attached to plants and have a battery life that lasts a few days and be easily recharged via cell phone charger.

Sources:

Adafruit Neopixel tiara

3D printed adafruit tiara with neopixels

fairy  throwies that are placed in trees that go along with a story to convince kids at a birthday party that they have called the fairies there.  I think I found this on instructables