Tips for Informal Family Portraits?

Just got asked by some friends to do some informal family portraits--going to a park and just playing around with a camera for a bit...any suggestions?

My thought is that at the beginning I'll have the husband, wife, and young child all interact, tell them some things like to try to stay smiling/happy, be close to eachother/touching often, etc etc, and then after that to try some slightly more formal/setup shots using a few SB800s....any tips? I don't know much about posing, I've read that keeping people out of shorts and sleeveless t-shirts is a good idea but beyond that I'm not sure if there are any do's or don'ts for their clothing choices, and I have a few ideas (based on my experiments and reading at strobist) about how to light them, but that's about it....

Thanks!

Bump...anybody?

I've done a few shoots like this and can add a bit from my experience.
First off, know the location. Have some ideas on what you want to achieve and where you'll be shooting. Start off posing a few pics and be sure to capture moments in between poses. Use natural props to your advantage, such as trees, bricks, walls, sidewalks, etc...(for example, play peek-a-boo with a young child behind a tree) As the shoot progresses, you and the subjects will loosen up. You'll start to connect and communicate better and this generally yields better pics....so save your best for last. Most importantly...have fun with it. Don't worry about the kids running off to chase a butterfly or whatever...just enjoy it and shoot away. That said, be sure to allow yourself plenty of time. I don't like to rush things so I make sure my subjects are in no hurry when booking the session. I would suggest giving yourself at least a good hour for the photo session....maybe more depending on the situation.