"Wir haben gegen die Kinder gespielt."
Translation:We played against the kids.
What makes you think so? I'm not exactly a native speaker, but it doesn't seem weird to me in any way...
Well, now that I read it again I take it means that it was a team game and the children were their adversaries, which makes sense. Initially, I thought it meant that the 'wir' referred to someone(s) playing while showing their backs to the children, and that wasn't very nice.
In any case, thanks for letting me notice.