I always have a hard time controlling my giggles around the kiddos, knowing that I need a straight face in order to act dissaproving, but finding it oh so difficult at times! When DD was 4 and it was a pretty wet and wild day outside, I discovered that I really needed to watch my language around her more carefully. She turned to me with an innocent smile and said "wow mum, it's really p*****g down outside!". Cue me trying to keep a straight face while explaining that was not the most polite way to phrase things.
Another one, this time from the school. As a teaching assistant, I often encounter issues where you know you can't laugh, but oh so want to. A few months back we were taking the kids down to lunch. One of the boys went running to the cloakroom to hang up his coat and then decided to jump his way back to us. As he jumped across the floor his trousers suddenly fell foul of gravity, and ended up around his ankles by the time he'd made it to the line with out him being aware. Needless to say, I had to turn away for a moment to compose myself!