It's really hard to locate, honestly. There were three or four candidates for that first driver that happened within ten minutes (the Paris trip/Kevin deciding he wants his family gone/deciding to leave him alone in the attic). It all depends on what you think the core 'idea' of the story is because any of them could work.
Everyone is criticising from the get-go, but it's also pretty clear that this has been going on long before the Paris trip was decided upon (the kids don't want to share a bed with Fuller/Buzz is criticising and mocking Kevin, etc.), which leads me to believe the actual 'inequity' doesn't begin until Kevin is left alone in the attic (which fits, given the OS is labeled as "An 8 Year Old Alone on Christmas").
Watching it in the context of a Mind story, it feels really weird, generally. Certain parts of it feel totally right (the 'you and I' moment lines up perfectly with IC Understanding -- Marley helping Kevin understand who he is, and influencing Kevin to understand what he needs to do -- and RS Present -- both of them connecting over their current family separation), but others feel really, really off (OS Signpost 4 of Memory is really light because 90% of the act is taken up with the Rube Goldberg house slapstick, except for one, maybe two scenes).
I'm still not 100% onboard with it being in Mind, but I'm remaining open-minded on that until I'm more well-versed in Mind stories.