I think the main issue you're facing is this: Are any of the champions mothers/fathers? Now, as far as sales on regular skins and things of that nature, I don't know many parents who still have time to play League after having a child so having a sale on these days would probably just be another excuse to have a sale. Unless human nature has changed recently, people don't want to sell things for less than what they are (perceived to be) worth unless there is a good reason. Considering there are very, very few parents that play League, if any, I can see why there was no sale.
The Happy Father Singed is an interesting concept and I actually like the idea of Father's Day Nunu, but you have to also look at the lore for these champions. If these champions really are fathers, are they REALLY going to be taking their kids to the Fields of Justice for an all-out slaughterfest? I think not. Now, that's not to say there aren't some bad parents in existence. There are TONS of people that let their 5 year olds play games that are rated M. Apparently the ESRB should just take the rest of their lives off because obviously their jobs aren't worth anything to most of America. I would just like to believe that the parents in the fantasy world of LoL actually have some sense of what is and isn't appropriate for children to witness.