Blac Chyna, Tyga Are Cool Again ... But Only for King Cairo's Sake

Blac Chyna and Tyga are willing to play nice with each other and even hang out together in public again, but only on one small -- yet growing -- condition ... namely, their son.

Sources connected to the former couple tell TMZ ... Chyna and Tyga's unexpected reunion this weekend for King Cairo's 5th birthday party at Six Flags was at least 2 months in the making, and mostly thanks to King's nanny.

We're told the nanny convinced the feuding exes to make peace during a 3-way phone convo. The impetus for burying the hatchet -- King was about to start kindergarten, which meant there would be upcoming activities requiring co-parenting.

Before the peace summit, our sources say Chyna didn't even have Tyga's phone number - but now the feeling is they're cool being near each other ... as long as King's third-wheeling.

If nothing else, opening communication lines should help 'em avoid planning disasters like doubling up King's birthday themes.