I am staying in Paris with a family of 6 -2 teenagers and 2 seniors in Aug 2010- would it be cost effective to rent a van and drive to Normandy for the day or take a tour from Paris.
If the total number is seven, such vans exist. Per- day rental is steep, but it's about the only way you can pull it off in one very long day.
Figure two and a half hours each way to Caen, depending on where in Paris you leave from and potential traffic problems during the morning and evening peaks.
A one-day van rental would cost in the neighborhood of $600 US. Do you intend to see the battlefields on your own? If so, then renting a van is the way to go.
However, if you wish to take a guided tour, then it might be more cost effective to take the train to Bayeux and back to Paris, taking an afternoon tour from there rather than a package deal from Paris. If you book well in advance, you can get €25 discount fare tickets for Paris-Bayeux and Bayeux-Paris.
Not all tour companies offer half-day tours. One that does is Overlordtour. Most people on this board think the best tour company is Battlebus, but they only offer all-day tours and they require their customers to spend the night before the tour in close proximity to Bayeux.