Welcome aboard, Pui Wan Yvonne! What a magical adventure you have in store!
If my cruise were stopping in Le Havre, I would have to splurge on a
Port Adventure to Disneyland Paris, because I know we wouldn't be heading there on our own anytime soon. If that's the same case with your family, I say go ahead and book it! There is quite a bit of travel time involved, about 3.5 hours each way. That's seven hours of traveling, but you'll also have about seven hours in the two parks there, which is plenty of time to enjoy lots of attractions and get a real feel for Disneyland Paris!
There are three options for this Port Adventure: On Your Own at Disneyland Paris, which includes just your park tickets and transportation, and then the Disneyland Paris Ultimate Day for Young Families VIP Tour, or the Disneyland Ultimate Day of Thrills VIP Tour for families with older kids. These VIP options will help families make the most of that one day at the parks with a VIP Tour Guide. Now, I haven't been to Disneyland Paris, but we have had the opportunity to have a VIP Tour Guide at Walt Disney World in Florida, and I can tell you from experience that they are truly wonderful! These guides will allow guests priority entry and FastPasses to many of the attractions, parades, and experiences in the parks. To truly maximize your time at Disneyland Paris, if the cost is manageable, this would be the way to go! But, I can only imagine that guests will be able to experience plenty even on their own, in the seven hours allotted for adventure at Disneyland Paris. Make the Paris only attractions your first stops then if time allows, enjoy some of the classic Disney attractions. Back onboard the Magic, grab dinner from a quick service spots which are open late and you'll be all set!
Have fun as you continue deciding on your Port Adventures, Pui Wan Yvonne!
Marcy