Hello, I'm an American living in Germany.

We just went to DLP over Thanksgiving and had a really nice time. It was our first Disney trip too, and I was really overwhelmed trying to plan it before I found this board. Everyone has been so helpful.
I got the best deal booking through the UK site, during a 40% off promotion. We booked about 2 months out. I checked the U.S., Ireland, and Germany sites as well but the UK site had the best price. I actually booked through Disney on the phone but the rep that I spoke with said it would be OK to book online, just leave the country as the UK when entering your address. It doesn't matter because they e-mail your confirmation.
I'm not sure about the prices (I think they are lower in May/Sept) but if the crowds are lower in those months I would take advantage of them. I would also go M-TH because I think the weekends are always busy.
I also have a 1 year old and a 4 year old.

The 1 year old was perfect, took naps in the stroller, etc. My 4 year old had attitude, pretty disappointing since we did the vacation mostly for her. I think she was just overwhelmed. You can check out my trip report for other details of our trip, but I only have the first day done so far.
I would suggest doing as many character meals as you can afford. We did the princess lunch at Auberge and Dinner and breakfast at Cafe Mickey. I think the key to meeting the most characters is to go early. We had lunch at 12 and dinner at 6 and they were both good experinces. Breakfast at 9:30 wasn't so great for Character interaction. Having the characters come to your table is so much nicer than standing in line, or having to push your way to the front, especially when you have young kids.
I looked at ryanair.com for flights (Rome-Paris). I didn't use your exact dates but looked in June and the prices were in-expensive. I also have no idea about the airports/transfers but I do that you can schedule a transfer through them. We have flown with them once from Frankfurt/Hahn to London and it was a good experience. Just beware, you will have to book your 1 year old on your lap, I tried to buy mine a seat and then had to re-book and get a refund. Pretty much a pain...
We have also been to Paris 2 times this year, so I can help if you have any questions with that portion as well.
And if you have any tips for Rome I would appreciate it! We are planning to go this spring/summer sometime.