Good point about travelling on the Metro...
Quite often, some of the group will get excited and rush in the first door they see, and the rest of the family gets left behind because the doors close too fast.
If this happens - and it usually does, at least once - make an agreement that the group on the Metro gets off at the next stop and waits against the wall, right under the clock. That's the easiest place to spot where you can find each other. Then everybody get back on the train together.
Other good points have been made - nobody can predict how, when or where the next attacks will come. But it's probable that we are all in this for the long haul - no matter where we live. I wouldn't advise getting too sombre about all this, since it can have a negative effect on sensitive people and young kids - try to lighten things up as much as you can, and involve everyone in the preparations for your trip.
Everybody should be outfitted with an "information" kit, to be carried securely on their person. This is extremely valuable, because if one of you gets separated and is injured, medical personnel and the police will know how to contact your family.
If you haven't thought about medical insurance, you should. Though costs are far less in France, you would want to be reimbursed for any expenditures you might incur.
If you are injured in France, you will be treated first, and payment questions will come later.
The security alert is at the highest possible level in France. More police have been ordered to report to duty. In the tourist areas, around the monuments, museums and shopping districts, you'll be well supervised. It's probably not a good idea to bring backpacks anywhere right now, unless it's absolutely necessary - and I can't imagine that it really would be. It ties up security lines and many places will not allow you to enter with backpacks, now. Try a canvas or nylon tote bag, instead - or have everybody carry his own.
If you have not already booked lodging, now would be a good time to stay in hotels. This is for the simple reason that there is always someone at the desk who will know what is going on and can give you good advice about possible areas to avoid and recommend things you might do, instead.
Some tourists are complaining about all the police and armed soldiers everywhere. I think this is a good thing. Most of us who live here don't even notice them anymore. If you're here for more than a couple of days, you won't mind them, either.