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.