We spent 10 days in Paris over Spring Break, also renting an apartment with a view of the Eiffel Tower (did you use Paris Perfect, by chance?). We had a great time but our kids said cruises are better!
Also, I think your kids are at an ideal age for the Fun Factory, especially the 8 and 9 year olds. I think Celebrity does a really good job with that age. Having a few other kids is nice but more kids does not necessarily make for a better experience, a fact which seems to escape those who try to direct all families to Carnival and Disney.
A few years ago, some of our empty nester friends gave us some valuable advice. They said, don't hesitate to travel as much as you can when your kids are in elementary and middle school. Because once they reach high school, it will become much more difficult. We have found this to be very true. Our oldest DD is a sophomore. It has been a real challenge to schedule trips around her school demands, including honors and AP classes, sports, extra curriculars like speech and debate, community service requirements, etc. The summer is also becoming filled with opportunities for her to do internships or attend sports or speech & debate camps. Our vacation desires now have to be worked around so many things that she needs or wants to do. Next year she will be a junior and we will start visiting colleges. So I don't regret any of our trips, even though each one may have had moments here and there of homesickness.
Caribbean Princess - Baltic Heritage - 6/15; Crown Princess - Mexico - 12/14 (the Xmas cruise where I broke my leg); Cunard Queen Mary 2 - Eastbound TA - 6/13; Celebrity Silhouette - W. Caribbean - 3/13; Sapphire Princess - Mexico - 12/12; Celebrity Equinox - E. Mediterranean - 6/11; Sapphire Princess - Mexico - 3/11; Queen Victoria - Mexican Getaway - 2/11; Mariner of the Seas - Mexico - 3/10; Sapphire Princess - Mexico - 10/09; Crown Princess - British Isles - 7/09; Celebrity Summit - E. Mediterranean - 6/08; Dawn Princess - Pacific Coastal - 5/08; Golden Princess - Mexico - 3/08; Diamond Princess - Mexico - 3/07; Celebrity Infinity - Alaska - 8/06; Caribbean Princess - 6/05; HAL Ryndam - Alaska - 8/96
Ruby Princess - Cabo Getaway - 12/17/16