I sort of did that in reverse last December. We went from Disney Paris to London, on two trains: first was SNCF from Disney to Lille, then Eurostar from Lille to London. It was blazing fast. The biggest problem we faced was the lack of storage space for luggage. We were in the middle of a three-week trip around Europe, and had a toddler with us. We didn't have a problem until the Eurostar. There is a little bit of space near the front and rear of each train car, but it filled up well before we were able to board. The aisles were narrow, and we had to drag our luggage and a three-year old the length of an entire train car on the way on, and then in reverse on the way off. Our luggage bumped into damn near every person in that car. We also needed to get off one stop before the final stop in London, and I think we were some of the very few people to get off there. It likely would have been easier if we had gone all the way to the end and let the car clear before exiting.
The train itself was fine. It didn't seem anything too fancy, but it was comfortable. And fast. If you have little kids, the pressure might bother their ears when it goes under the Channel. Our son was fine, but the couple across from us had an infant who cried almost the entire time, from ear pressure.