LeHavre is one of the ports for Paris. I would advise either purchasing a transfer from the ship which will provide your transportation from LeHavre to a central spot in Paris or selecting a guided tour in Paris. (It's too far to go on your own -- it is at least a 2 hour drive from the port to Paris. Make sure your kids are well rested for a long, but thrilling day!) If you do the transfer, do a little research ahead of time and try to get a map of Paris so you can sightsee on your own. Sometimes kids like to go at their own pace rather with a guided group.
I helped edit a great guidebook called, "Cruise Vacations with Kids" (by Candyce Stapen, Prima Publishing) which offers excellent suggestions on what to do with kids in various ports. I wrote a section on European ports for kids and LeHavre/Paris is listed there. You may want to pick up a copy and research the other ports on your itinerary.
Have a wonderful cruise.
Luisa Frey Gaynor