[Note: this festival is outside of Tokyo.]
In the children's park known as Mt. Fuji Children's World in Shizuoka, you can catch a two-day camp festival. Although the focus of the festival is music, there are lots of kid-friendly activities to enjoy, including movies, workshops, canoeing, horseback riding and even a petting zoo.
The music lineup is a really unusual selection, including the brilliant Brazilian composer and multi-instrumentalist Hermeto Pascoal, soundscape artist Theo Parrish and phenomenol guitarist Mateus Asato performing in a trio, plus many more.
The movies being screened include the Studio Ghibli flick 'The Tale of Princess Kaguya', the 2014 comedy 'Chef', and a few others. Check the website for access from Mt Fuji Station.