September with two youngsters

You need to research how long the train would be, if it is long hours can't imagine 3 and 4 year old being able to handle it. Also would think you would want two nights in Paris, one would be just too short in my opinion.

Personal opinion , you say they just moved there, thats enough adjustment for two tots ,, a whirlwind trip to another country,, well, I wouldn't do it.

Where are you coming from in Italy? That might make a difference. If it's Rome, and you really feel it's a good idea to take a two kids under the age of 5 on a trip just to get to a playground for a few days, then by all means you should fly. I think you should consider sending your daughter away to a hotel for a few days while you get to spoil your grandkids in their own home. They'll get you, their toys, their own beds, and still have access to gelato and pizza. What more could a kid of that age want?

They surely have not seen all of Italy? There is no train journey to Paris and return to any point in Italy less than six hours each way - two days lost travelling, door to door.

If there were no children involved, a few days in Paris is not a problem (I'd fly from most places in Italy, but overnight trains would also work if you don't mind them). But we're talking about two children who are so young that they won't know the difference between Paris, Columbus, and any place in Italy. Schlepping them through train stations or airports, for a short jaunt, doesn't sound like fun, particularly since they've just been schlepped from America. And if they've all just moved to Italy, what do they need to "get away" from? I could certainly see this being an issue if they've been there a while, but you said they haven't. If everyone does want a change of venue, I think, as Neil said, that another place in Italy would be much easier. If you want to see Paris, you should think about doing it on your own, before or after you see your daughter. Last year, my sister and her husband took her 3 year old and 5 year old from New York to Montreal, where they rented an apartment for a week. They had hoped the kids would be fascinated by the differences, particularly the fact that people were speaking French. The kids loved the airplane ride (even though they get motion sickness, they can't wait to go on another plane, as they were so fascinated by it). But they didn't really appreciate any differences between Montreal and New York. The parents did, so the trip was a success. But the lesson they learned from this is not to go to a "foreign" place for the kids, until they're a bit older (again, in this case the parents liked the "foreign-ness" and the kids just enjoyed the playgrounds, zoo, etc like they would have at home). The other reason the trip was a hit was that it was a week, so everyone could relax and feel "moved in" to the new location. With just a few days, you wouldn't be able to do this.