Subject, Skills, and Grade Level
Empathy, and really thinking about why we have the relationships we do with others. Haven't been fully play tested with an array of age ranges yet, but I think with parental help and guidance, you could go as young as 4 or 5.

Time & Players
15-60 minutes, depending on how in-depth you go. If used as a discussion tool, can be any number of players, but recommended is 1-3.

Rules, Tools, & Assessment
As a solo journaling game, no GM is required. As a discussion game, a parent or older person to guide discussion would be useful. All you need is a deck of playing cards (jokers removed) and 3d6.