Muji's crossing over into cult territory, but at least it's a happy, well-designed, logo-free, ecologically sound, economical cult.
Muji Summer Camp has two sessions, and you already missed one. [The next one is Aug. 23-25, 3 days/2 nights.] It sounds basically like an archetypal summer camp, where elementary school-age kids growing up in an overwhelmingly urban environment spend a little time in nature, doing outdoorsy things and sleeping in tents. Muji tents [d'oh, they're gone.].
The website's in Japanese only, duh, but you can get a sense of things by clicking around.
It's 39,000 yen per kid if you're using the Muji campground next door, 44,000 yen if you're not. Yes, there's a Muji campground, too. And they rent you everything you need, in little 4-person kits.