So far I have progressed quite a ways through the game and the random encounter with bryan wilks around the super duper mart area has not occured. I've even been to grayditch and he's not there. I read somewhere that there's a bug where he can be attacked by npcs. When I went to the Super Duper Mart the first time I noticed a random encounter had ocurred because some ghouls who were trying to get to Underworld had just killed the raiders there. Either that or my other guess is he was killed by the Enclave Soldier by the farmhouse up the road from the Super Duper Mart. Either way, I'll be pretty bummed if I can't do the quest "Those" and there are always annoying Fire Ants around Grayditch.

Kids cannot die in Fo3 he around there..somehwere you he may have feel through the map with a glitch , but most likely you just keep missing him.