An ongoing game of DW I'm running for my 3 boys, Michael (16), James (12) and Gabriel (10). Michael is playing a Paladin, James is playing a Wizard, and Gabriel a Ranger. The world has gone from Typical Pseudo-English Medieval to Pseudo-Middle Ottoman Empire to Heart of Darkness in Hell Forest, and they're having a blast.
Gabriel is the only one who's had his character show any, well, romantic interests, at all. By loudly announcing, 'I want my character to find a girlfriend!' So, I gave him one – an elven barmaid, who helps them out when a demon got loose from an artifact, while they were in the tavern (whoops!). She's not a terrific fighter but can hold her own, and has a few secrets to be discovered. Good basic interpersonal relationship roleplay stuff, appropriate for a 10 year old.
So, a few more sessions in, and they've gone through the ancient portal (Stargate!) from Sorta-England to Sorta-Anatolia. At some point, Michael asks Gabriel (he of the NPC girlfriend) "You've already got a girlfriend, what more do you want?"
Gabriel puts his arm down on the table, leans over with a big grin and says, "Kids!"
(Oh, yeah, Dungeon World is a fantastic system, too. I highly recommend it.)