Henry would know before they did! Emma would be all like “Henry…there is something your mom and I need to tell you,” but Henry would just roll his eyes and respond with “….moms, I’m 13 not stupid”
Henry is the type of little shit who would actively play parent trap. like he knows regina loves emma and emma loves regina and he is just SO tired of them always giving each other dumb heart eyes without knowing they are in love with each other so one day he tricks him moms into a romantic setting, telling them hook and robin are waiting ~ all the while calling Hook and Robin and being all like “I wanna hang out with my step dads!!!” just to get them out of the way so regina and emma can FIGURE OUT THAT THEY ARE LIKE MADLY IN LOVE WITH EACH OTHER.
all he wants is for Regina and Emma to be happy. I mean IN CANON he hates Hook and doesn’t really care about Robin that much besides thinking he’s sort of cool. that kid would be absolutely ecstatic once those nerds got together.