I've not been to too many kids' parties, but like I said, to me the norm is to unwrap the presents at the party, but not necessarily play with them (maybe with a couple of exceptions, like a one-piece truck or something). I think that's something that could easily vary by region/social circle, though, and even the particular child--the post about young children being overstimulated by gifts and just wanting to rip into the next one reminded me of my cousin's kid's parties. The child was three, four, and five, I think, when we went to her family-only parties, and was so terribly rude and greedy (though at that age I blame the parents for not teaching/correcting her) it just made me ill to watch. She got these huge, expensive, multi-part gifts from her doting aunts and uncles, and as soon as she got the paper off she would be like, "Another one!" in a demanding tone. Then at the very end her parents would try to make her say one big "thank you" to everyone and suddenly, she was shy
But gift-opening was a major "activity" of the party, indeed the only
activity aside from nibbling snacks and cutting the cake, so we all had to sit around her and watch.
Like a lot of things I don't think the activity itself (opening or not opening the gifts at the party) is rude, but it depends on how well it's managed, which is where the host and, if applicable, parents of guests come in. I could see how not
opening the gifts could be made rude, as well--say, if the host basically grabbed the gift and stockpiled it with a greedy glint in their eye, then begrudgingly gave the guest a couple of pretzels and tried to get rid of them right away, with no games/crafts/toys/etc. for the kids to play with.