Rob Kardashian Channels Inner Stepdaddy

8:05 AM PT -- On closer inspection ... it appears he's buying an organizer for King Cairo ... a possible sign Blac and her son may be shacking up with Rob long term.

Rob Kardashian did a little bonding with King Cairo on a trip to Toys "R" Us ... thing is ... Blac Chyna was nowhere in sight.

We've seen pics of Rob playing with Tyga's son ... and sources tell us Tyga's cool with it ... but taking the kid out alone?

As for whether Rob's alone time with King Cairo could incite Tyga ... well, maybe he should call Russell Wilson.