I’ve seen this happen a few times, but I know it happens quite often. I’m an equal opportunity seat-not-giver-upper and would never do that unless it was someone I’m dating. What’s the point? They’re not going to do anything but say thank you. It reminds me of what Gwyneth Paltrow said when she was wearing her fat suit for “Shallow Hal.” It was something to the effect of “It felt so awful, no one was paying attention to me at all!”

I’d much sooner give the seat to an elderly person. I’m somewhat on the fence about kids because I reason that they have tons of energy. Plus the parent would probably end up doing this crap where they sit down and put the kid on their lap until the kid realizes it’s not all that uncomfortable and ends up standing anyway.