Q. If my child acts up at a playdate, do I punish him then and risk ruining the fun for the other kid? If I wait, the discipline won't make sense.
A. In all the excitement of having a pal over, many a child has temporarily lost his mind and acted a fool. But punishing your kid way after the fact will most certainly be useless because, as you suspect, he probably won't remember his transgression, much less why he's in hot water.
That's not to say that he should be allowed to go hog wild when company's over. It just means approaching the situation a little differently. With my girls, I lay down the law before friends arrive, and while they're here, too. I make it clear to them that the rules don't change just because they have guests, and that they're to be on their best behavior -- to set an example for their visitors and to stay out of trouble.
Somewhere in the conversation, I remind them that they really don't want to get embarrassed in front of their friends, but that I will discipline them if they get out of hand -- no matter who's listening and watching. And I let the other kids know the ground rules, too: No running through the house, no tossing balls in the living room, etc.
Even so, they have their moments. And at those times, I try to stop whatever's going on, scold when it's called for, redirect when it's not a big deal, and let the kids do what they do: have fun.