That's alot of questions. I'll try to answer as many as possible!
Should we take the train from Italy to Paris? Never done it but it's possible. Look here:
www.raileurope.com
If we drove, we would need snow tires--right? Probably not, never done the journey myself but people don't really use snow tyres in europe.
The Disneyland hotels don't look too expensive, but how will they compare to a Priceline hotel in Paris? The Disney hotels can be cheap or expensive. Some days in December are peak season so look around. I've found that you can get some good room only rates on
www.expedia.de. Some of the good-neighbour hotels are very good value, especially the holiday inn (book directly through their website). Priceline has some good deals for Paris. It all depends on if you want to stay in the magic or in Paris I suppose. Hotel rooms in Paris are VERY small compared to American standards.
If we stayed in Paris, could we get to the parks via train? Yes, it's very easy. It takes about 40 minutes on the Orange RER line. Just make sure it's the correct train when you get on as the line splits in two!
Is there some hotel you would recommend? In the resort Disney's Cheyenne is the best value. The holiday inn just outside is nice too. In Paris I've stayed in a lovely hotel called Hotel de la sorbonne. The rates are very good but the rooms are tiny. Use
www.tripadvisor.com to find a hotel to suit you. It has reviews of almost every hotel in the world!
Is there food available that my very picky grandson will eat such as burgers or any kind of fish? Disney food is American food. Burgers, burgers burgers!!! In Paris there is a wide variety of food. They have many good pizza places as well as Planet Hollywood and the Hard Rock Cafe. Food should not be a problem!
I had one year of high school French and my grandson is currently taking French--will we have enough understanding to get by? I can't speak a word of French and I always get by. Most Parisians have a decent grasp of the English language. Most Disney staff speak English fluently.
Are the packages at Disneyland good value? The packages differ depending on where you're from. In England we're currently offered one extra night on stays in early december. The package prices all include buffet breakfast and park passes. If you can get a deal they can work out cheap. Otherwise I don't find them to be particularly good value.
Hope that helps!