Take your pick..... Oshawa (LSE), Hamilton (LSW), Mount Pleasant (Kitchener Line), Aurora (Barrie Line), Mount Joy (Stouffville Line)
Depending on how old your kids are, I would try Union to Pickering, get off and take the pedestrian walkway to Pickering Town Center, then walk back again and board a Union bound train. Kids only really enjoy the first couple stops and then they get bored....give 'em the walk to burn off some sugar, or treat them in the food court.
Go Schedules page is your friend.
- Paul
PS - (shameless advert) Or - take them to one of the heritage museum railways in the area.