The child of Oliver Queen has been a dangling plot thread ever since the reveal back in season 2, and we got another reminder of it when he bumped into the mother in The Flash episode Flash vs Arrow. Obviously Oliver has no idea yet, but its probably not going to stay that way. Many fans believe that the child will turn out to be Connor Hawke, who is Olivers son in the comics and ends up taking on the Green Arrow name. Even if Olly were to meet his son in the show though, he would only be around eight years old in the current timeline. Not exactly old enough for a vigilante. The theory, though, is that Hawke (or whatever name he gets in the show) will end up travelling back from the future and meet his father, Green Arrow to Green Arrow. That would be stretching things pretty far, and would certainly be a mind (and time) bending development. More likely thought is that Olivers son does turn up, as a child, presenting further problems for his father in his efforts to juggle being Oliver Queen and the Arrow.