Sure, you can do that. You can buy your tickets ahead online (eurostar.com) and get them a lot cheaper than at the station. I believe the Eurostar tickets go on sale 120 days ahead, but I'd have to look that up. You can easily find out by looking at their site. Take the earliest possible train out and the last one back. 2nd class is fine. If you have your day planned, you can see quite a bit -- enough to make you wish you had a lot more time. Both London and Paris are wonderful, but what contrast! I'm not at all sure I'd go for the hop on hop off bus. I personally would concentrate on seeing the 3 or 4 things that are the most iconic to you, and see those, such as the Eiffel Tower, Arc de Triompe, Champs Elysee (that's not spelled right), and Notre Dame. For example, you could take a taxi from the train station (which is Gare du Nord) to Notre Dame. Then you could spend your whole day working your way along the Seine from that area up to the Arc and on to the Eiffel Tower. You can also get a cab along the way if you tire of walking. With walking, your son will get a feel for Paris. Even if the line to get into Notre Dame is very long, it moves very quickly. Go to the top of the Eiffel Tower if the lines aren't too long. Just seeing it from the ground is awe inspiring though. You can climb to the top of the Arc. Eat at a sidewalk cafe. Get ice cream and watch the people go by. I think its going to be a day your son will remember a lifetime. There is no place anywhere like Paris. Go with a plan, but don't worry if you don't follow it. Just enjoy being there. Go with some Euros or find a machine quickly so you'll have some cash. Don't worry about having some Euros left over. You'll need them on your next trip to Paris. No one can see Paris and not long to return and return and return. I want to go! (and we just went in June).