I would not recommend a calling card, unless you enjoy standing on random street corners and shouting over traffic noise to make calls. And with the pervasiveness of cellphones, there are far fewer pay phones around.
Here is my suggestion. First, have the grandparents get on a cheap international plan; they can call France for pennies a minute on most plans. Next, you need a cellphone. If you have a T-Mobile or AT&T quad-band phone such as the Razr and get it unlocked, it will work in Europe. Otherwise see if a friend has an old one they can loan you for the trip (unlocked, of course). Or you can buy one on eBay for around $10-$20. Then in France buy a SIM card. With a SIM card calls to the US are about 50 cents/minute, and incoming calls are free. So when you have time to talk, simply call home (or send a text message, even cheaper) and have them call you right back; then talk as long as you want.
As to expenses, Ricks site says to plan for about $100 a day in expenses per person. I think that is a little high, but I would think $100 per couple would be reasonable, more if you're a shopper. Remember you're buying 1/2 your meals, your own wine at the tour-paid dinners, snacks & water, admission to places not covered by the pass they give you, etc. I would not go to my local bank and change this much money. On the other hand, many people (me included) like to show up in Europe with some Euros in their pocket to buy a snack, take the Metro, etc. I would suggest changing about $100 into Euros to get you started, then use ATMs for the rest.
Wall sockets in France use a 2-prong round type plug; you can find a picture of them online so you see what I'm talking about and buy them at many travel stores, amazon, etc. Make sure your camera charger says it works on 120-240V or you will fry it.