Adult party - I would think when they are given.

With my DS we generally wait until afterwards. Only because at that age he is just as likely to open it, and either say "I don't like that" or cast it aside. It's not that he's an ungrateful kid but when they're really young they don't quite understand that they could be hurting someone's feelings. Also, with kids parties they tend to be so chaotic there simply isn't time.